Mozilla Release Notes

Version 157.0, first offered to Release channel users on September 29, 2026

New

Firefox's biggest visual refresh in years is now available, with a modernized look across the browser toolbars, sidebar, menus and individual features, built with our updated design system. It also includes:

• New built-in themes with light and dark versions, to personalize the look of Firefox in one click.
• A new compact mode that reduces toolbar and tab spacing to leave more room for web content, useful on smaller screens and in split-screen layouts.

WebRTC video calls can now use hardware AV1 decoding on devices that support it, instead of always decoding AV1 video in software.

Firefox now shows a warning in the site information panel (the shield icon in the address bar) and in Settings when it has been set up to record its encryption keys, which could let other software on the computer read encrypted web traffic.

Firefox Suggest now offers suggestions in the address bar in Austria, Belgium, Czech Republic, Denmark, Finland, Hungary, Ireland, Luxembourg, Netherlands, Norway, Poland, Portugal, Slovakia, Spain, Sweden, and Switzerland. They can be controlled in the Firefox Suggest settings. Learn more.

Fixed

Fixed video from phones and tablets appearing sideways or upside down in WebRTC calls.

Fixed the address bar searching for the typed text instead of opening the selected suggestion when a page was still loading, such as shortly after Firefox started.

Fixed some HDR videos encoded in 8-bit color formats looking dull and gray on Windows instead of displaying in HDR.

Improved audio and video synchronization when repeatedly changing the playback speed of videos, for example on YouTube.

Fixed the Vertical Tabs sidebar not reappearing when the cursor moves to the edge of the screen in full screen mode.

Fixed an issue on Windows where making a video full screen would leave the Windows taskbar showing over the video.

Various security fixes.

Changed

Pressing Tab now moves focus straight to the text field of the address bar and search bar instead of stopping on the search engine button first. The button can still be reached with Shift+Tab.

Amazon is no longer included as a built-in search engine. It can still be added as a custom search engine in the search settings. Learn more.

The updated sidebar is now enabled for everyone. Anyone still using the previous sidebar will keep a similar layout, with the panel switcher at the top of the sidebar. Additionally, the “Show sidebar” option has been removed from Settings.

While we continue to refine the new design, the previous sidebar version can still be restored by setting sidebar.revamp to false in about:config. This also turns off vertical tabs. The preference will remain available until the end of 2027.

Web page alert(), confirm(), and prompt() dialogs, the color and date pickers, and form autocomplete drop-downs now match the page's light or dark color scheme.

Enterprise

You can find information about policy updates and enterprise-specific bug fixes in the Firefox for Enterprise 157 Release Notes.

Developer

Developer Information

Web Platform

Firefox now supports the at-rule() function for @supports. This allows authors to detect browser support for specific at-rules, such as @supports at-rule(@scope).

Firefox now supports the chain value for the CSS overscroll-behavior property.

Community Contributions

With the release of Firefox 157, we are pleased to welcome the developers who contributed their first code change to Firefox in this release, 21 of whom were brand new volunteers! Please join us in thanking each of these diligent and enthusiastic individuals, and take a look at their contributions:

• Alexander Gankov: 2033184
• Amadi: 2053549, 2053553, 2065302, 2066478
• Andrew: 2031465
• Caleb Pickard: 2058970
• David Torcivia: 2057758
• dchen705: 2065655
• Ekerin Agboola: 2064541
• Giddeon Card: 2067328
• gustavokrm: 2051536
• Hanzala Sohrab: 2068111
• Ivan Smagin: 2061540
• Jieke Jiang: 2067179
• Nazeer Ahmed Shaikh: 1703780, 2064544
• Noah Varghese: 1625924, 2021799
• perdomojuan755: 2069230
• Raúl: 2066877
• Rév O'Conner: 1594489
• Samuel Mbabhazi: 2054228
• sfshah: 2004273, 2067302
• Valerie Racine (:v-racine): 1849963
• Xiaosen Zhuang: 2069280

Changed

RNP command-line utilities are no longer included with Thunderbird

Thundermail built-in add-on updated to v2.0.16

Fixed

Ctrl+Shift+K could fail to open the Quick Filter UI

Nested S/MIME signed parts could fail to render

Thunderbird could hang when waiting for IMAP server greeting during account setup

Gmail OAuth2 authentication could intermittently fail on Windows

AppUpdatePin policy did not prevent Thunderbird from updating beyond pinned version

"What's New?" linked to Release channel notes instead of ESR release notes

Security fixes

As always, you’re encouraged to tell us what you think, ask for help, or file a bug in Bugzilla

What’s New

• Enterprise policy DisableChat disables Thunderbird Chat functionality
• Enterprise policy DisableFileLink disables Thunderbird FileLink functionality

Changed

• Preference mailnews.headers.minNumHeaders has been removed
• Allow viewing remote content in OpenPGP messages encrypted with integrity protection
• RNP command-line utilities are no longer included with Thunderbird
• IMAP/POP manual config port field is now optional
• Thundermail built-in add-on updated to v2.0.16

Fixed

• Ctrl+Shift+K could fail to open the Quick Filter UI
• Message list could show the wrong sender for messages
• Status bar could remain active and cause 100% CPU usage after activity ended
• OpenPGP discovery failed to find replacement keys after a key was revoked
• Valid RSA OpenPGP keys could be rejected, preventing encryption
• Sent messages could silently fail to save to the IMAP Sent folder
• Sending message did not fail gracefully with malformed references header
• Thunderbird could hang when waiting for IMAP server greeting during account setup
• Account Hub account creation could fail with malformed URI
• Custom OAuth Endpoint Host incorrectly required a full URL instead of a domain
• Message filters could falsely match when evaluating search term failures
• CardDAV address books could fail to sync when no password prompt was needed
• Yahoo emails appeared blank with MIME handlers disabled via mailnews.display.disallow_mime_handlers
• Pending moves within the same IMAP account could make messages disappear
• Inline images could disappear after editing and resaving drafts
• SMTP OAuth2 failed when large access token exceeded command line length limit
• Gmail OAuth2 authentication could intermittently fail on Windows
• Exchange NTLM authentication failed when updated passwords were not saved
• SMTP AUTH LOGIN could fail after username challenges and close the connection
• AppUpdatePin policy did not prevent Thunderbird from updating beyond pinned version
• Recurring events could appear after their configured end date
• CalDAV task body could remain outdated after syncing changes from another client
• CalDAV invitations could fail when accepted before the calendar synced

Visual and UX improvements

Security fixes

As always, you’re encouraged to tell us what you think, ask for help, or file a bug in Bugzilla

Version 156.0.1, first offered to Release channel users on September 22, 2026

Fixed

• Fixed the NVDA screen reader not announcing address bar buttons when the mouse pointer moves over them. (Bug 2069276)
• Fixed elements inside a link not showing their CSS :active styles while being clicked. (Bug 2067272)
• Fixed Firefox becoming unresponsive on some pages that use CSS anchor positioning. (Bug 2070171)
• Fixed Firefox windows on macOS 27 immediately returning to maximized after being restored by double-clicking the title bar. (Bug 2066632)
• Fixed Firefox leaking system handles on Windows as content processes were started and stopped. (Bug 2069644)

Changed

• Pressing Tab now moves focus straight to the text field of the address bar and search bar instead of stopping on the search engine button first. The button can still be reached with Shift+Tab.

Reference link to 156.0 release notes.

Fixed

Detached attachment links were truncated when folder path contained &
Save All, Detach All, and Delete All failed after deleting one attachment
Blank 1970-dated messages could appear from incomplete message file copies
Forwarded messages could display incorrect encoding
Manual Get Messages failed after startup until toggling offline mode
Thunderbird could crash on shutdown while waiting for OAuth2 password entry

Security fixes

As always, you’re encouraged to tell us what you think, ask for help, or file a bug in Bugzilla

New

• Firefox on macOS can now be set to open automatically when the computer starts up, from the Startup section of Settings.
• Localized Wikipedia and occasional sponsored suggestions in the address bar are now available in France, Germany, and Italy. These suggestions can be turned off in Firefox Suggest settings.

Fixed

• Fixed an issue where dragging an image to a new position in a rich-text editor could replace it with a long line of text instead of moving it.
• Fixed high-sample-rate FLAC audio in MP4 failing to play on sites such as Bilibili.
• Fixed subtitles not appearing in the Picture-in-Picture window when they are turned on in the video player after the window has already opened.
• Fixed bookmarks moving out of their folder when the folder and the bookmarks inside it were selected and dragged together in the Bookmarks sidebar.
• Fixed the find bar becoming permanently unavailable in a tab after reversing the panes in Split View.
• Fixed some sites failing to load when DNS over HTTPS is enabled and the site does not support HTTPS.
• Fixed Firefox losing all network connectivity for some users with the built-in VPN enabled.
• Fixed an issue on Windows where Firefox could block an auto-hiding taskbar from showing.
• Various security fixes.

Changed

• Firefox's built-in PDF viewer now starts up to 45% faster.
• Improved memory and CPU usage when Firefox displays large JPEG images scaled down to fit a page.
• On Windows devices with ARM64 processors, WebRTC video calls may now use hardware H264 decoding instead of falling back to software.

Enterprise

• You can find information about policy updates and enterprise-specific bug fixes in the Firefox for Enterprise 156 Release Notes.

Developer

• Developer Information

Web Platform

• text-box-trim now uses the font metrics of a ::first-line pseudo-element when one applies, matching other browsers and the specification.

Community Contributions

With the release of Firefox 156, we are pleased to welcome the developers who contributed their first code change to Firefox in this release, 10 of whom were brand new volunteers! Please join us in thanking each of these diligent and enthusiastic individuals, and take a look at their contributions:

• Aaron Ma: 2063818
• Dominik Bay [:eimann]: 2066736
• ExplodingJoysticks: 1728637, 1890400, 1910664, 1928343, 1944206, 2000658, 2003000, 2059911, 2061541
• Gopalarathnam Venkatesan: 2003172, 2042426, 2062096, 2062952, 2062989
• Hugo Osvaldo Barrera: 2063808
• Kana: 2062133
• Kerissa Grubb: 2042422
• Louis-Philippe Manette: 2062972, 2063867
• Seunghyun An: 2063033
• William McDonald: 1034964

What’s New

• OpenPGP key properties can print debug information to the Error Console
• External browser login flow enabled for Yandex
• Custom OAuth support for issuer ID and client secret field for IMAP and POP3
• Custom OAuth support for POP3
• Enterprise policy DisableUpdateSettings prevents users from changing update settings
• Enterprise policy DisableDataCollectionSettings prevents users from changing data collection setting
• Enterprise policy DisableMessageForwardingFilters prevents message filters from automatically forwarding messages

Changed

• Preference mail.offer_experimental_features renamed to mail.experimental_features_settings.enabled
• Thundermail built-in add-on updated to v2.0.11

Fixed

• Detached attachment links were truncated when folder path contained &
• Opening a PDF attachment in a tab displayed empty tab
• POP3 folders with names over 55 characters could become duplicated after upgrading
• Folders with invalid filename characters could show hashed names if cache was lost
• Save All, Detach All, and Delete All failed after deleting one attachment
• Renaming folders could reset folder settings
• Subscribing an IMAP folder and subfolder together failed to subscribe the subfolder
• Some newly fetched POP messages were not marked with new star after startup
• Sorting after deleting the only child message removed the parent from view
• OpenPGP signing failed with external GnuPG when the primary key could not sign
• Remote content could load in encrypted messages opened from files
• Sent message with expired S/MIME certificate wrongly showed untrusted CA warning
• PDF attachments from compose windows could wrongly be detected as HTML
• Saving unconfirmed recipients could create invalid address pills with >>
• External browser OAuth failed when a browser preconnection closed first
• Users could not set up custom OAuth for Exchange accounts
• Pressing Enter on Windows system tray icon did not restore Thunderbird window
• Inline attachments incorrectly changed the message channel content type
• Message loading was slow while discovering large numbers of IMAP folders
• SMTP authentication failed when usernames contained non-ASCII characters
• Unsupported EWS archive folder prevented all mailbox folders from syncing
• Manual Get Messages failed after startup until toggling offline mode
• Case differences in CalDAV email addresses caused duplicate invitations
• Accepted CalDAV invitations could be saved to the wrong calendar

Visual and UX improvements

Security fixes

As always, you’re encouraged to tell us what you think, ask for help, or file a bug in Bugzilla

Version 155.0.1, first offered to Release channel users on September 4, 2026

Fixed

• Fixed Firefox becoming unresponsive on some pages that use CSS blur and backdrop filters together. (Bug 2068836)
• Fixed the sidebar not reappearing after restarting Firefox when the "Hide tabs and sidebar" option was turned on. (Bug 2065431)
• Fixed some profile icons not being displayed in the account and application menus. (Bug 2064729, Bug 2067734)
• Fixed the current device being listed among connected devices in the account menu. (Bug 2059763)

Reference link to 155.0 release notes.

Unresolved

Standard dynamic range (SDR) video in some formats can appear washed out on macOS. The issue is under investigation and a fix will ship in a future release. (Bug 2068961)

What’s New

S/MIME popup shows signature, digest, encryption algorithms and key sizes

Default address book for manually added contacts configurable with mail.addr_book.add_item_default_uri

Enterprise policy DisableQRExport can disable QR export

OAuth add-ons can override built-in settings for selected domains and hostnames

Manual account config streamlined with Exchange, Graph, and custom OAuth support

Custom OAuth support for IMAP and SMTP servers

PKCE and external browser fields added to custom OAuth settings

Enterprise policy DisableExperimentalFeatures for disabling experimental features

Fixed

Unread count dock badge disappeared on macOS restart when no mail window opened

IMAP tags did not sync automatically across clients

Embedded SVG images appeared broken when Display Attachments Inline was disabled

Holding arrow keys failed to smoothly navigate the folder pane

Mark and Tag menu bar items were disabled in search results

Quick Filter ignored address book display names shown in message lists and headers

Attachment filters could disappear or return no results in global search

Body search failed for quoted-printable messages ending on a soft line break

Failed encrypted send did not preserve draft

Failed account setup left an incoming server that blocked account recreation

Removing an account did not remove its OAuth token from the password manager

Account Hub prompted for calendar credentials immediately after successful login

Filters could copy empty messages to Local Folders during startup mail download

Closing message filters wrongly warned completed newsgroup filters were running

Autoscroll icon was missing directional arrows in scrollable messages

Apple Mail text attachments may not appear in MIME messages

Forwarded messages could display incorrect encoding

Thunderbird could crash due to improper database memory handling

Thunderbird could crash on shutdown while waiting for OAuth2 password entry

Thunderbird could crash while loading mail folders

Custom OAuth could not be disabled

Legacy IMAP tag changes were not sent to the server and reverted after refresh

Deleting an Exchange account retained the outgoing server settings

CalDAV calendars with offline support could stop fully synchronizing events

Offline cached ICS calendar changes were silently lost after reconnecting

Thunderbird upgrade disabled composition toolbar

Thunderbird upgrade disabled Settings menu for versions before macOS 13

Windows jump list menu no longer functioned correctly

Visual and UX improvements

Security fixes

As always, you’re encouraged to tell us what you think, ask for help, or file a bug in Bugzilla

New

Smart Window is now available for all users in the USA, Canada, and France to try out! Additionally, Firefox 155 includes many new enhancements:

• New personalized resume conversation starters help users quickly return to topics they’ve been working on.
• Assistant now responds much faster with Exa web search source citations to provide accurate, trustworthy web grounding in roughly 1/5 the time.
• Organize Tabs now suggests better groupings of your related tabs and gives each group a clearer, more descriptive name.

This feature is part of a progressive roll out.

Firefox now shows how many trackers it has blocked in the address bar, making built-in privacy protections easier to see while browsing. The count can be hidden in settings without turning off tracker blocking.

This feature is part of a progressive roll out.

Nintendo Switch Pro and compatible aftermarket controllers now work fully in web games on Windows, with the directional pad and analog sticks correctly mapped.

Containers can now be reordered in Settings, and the new order is used wherever containers are listed.

The Translations feature now supports the following languages:

• Marathi
• Urdu

Fixed

Fixed text disappearing in Microsoft 365 Word Online and other web apps on Linux when typing with certain keyboard layouts.
Fixed an unnecessary paste confirmation appearing when using the keyboard shortcut to paste without formatting in Google Docs.
Firefox now recovers from the toolbar and tab strip being left unresponsive when the platform fails to end a tab drag session.
Fixed text inside buttons not being selectable or copyable, which prevented copying direct messages on X and comments on YouTube.
Fixed live audio streams such as internet radio stopping when playing in the background.
Fixed the sound indicator disappearing from a tab that was moved to another window, leaving no way to see or mute what was playing.
Fixed Firefox preventing Linux systems from going to sleep after a long browsing session.
Fixed full screen content being letterboxed on MacBooks with a camera notch, most visibly when watching video.
Fixed arrow-key and Page Up/Down scrolling not working on some pages with embedded content from other sites.
Fixed characters from some less common scripts appearing as empty boxes on web pages on Windows.
Fixed WebCodecs VideoEncoder hanging on macOS when encoding a VideoFrame in NV12 format.
Various security fixes.

Changed

Firefox now limits CSS to 75 levels of nested blocks and functions, preventing crashes that extremely deep nesting can cause under some circumstances. More deeply nested rules and values no longer parse.
Firefox now uses firefox-portal-detection.com instead of detectportal.firefox.com to detect captive portals and check network connectivity, so network allowlists referencing the old domain need updating.
Firefox now opens mailto: links only in response to an explicit user action, so pages can no longer launch the default mail client unprompted.
PDFs smaller than the paper size can now be enlarged to fill the sheet when printing, by setting the scale above 100%.

Enterprise

You can find information about policy updates and enterprise-specific bug fixes in the Firefox for Enterprise 155 Release Notes.

Developer

Developer Information

The Rules View now offers toggles to emulate the prefers-reduced-motion media feature. Emulation options have also moved into a new panel behind the at-rules (@) button.
The Debugger in Developer Tools now has a keyboard shortcut, Ctrl+Alt+B (Cmd+Alt+B on macOS), to enable or disable all breakpoints at once.
The JSON Viewer now displays JSON Lines (JSONL/NDJSON) documents, including inline errors for invalid lines.
Fixed the Storage panel showing cookies from private windows, containers, and other partitions alongside the inspected page's own cookies.
Uncaught non-Error exceptions now list their property names in the Console instead of showing only Object.

Web Platform

The CSS attr() function now supports values of any type, so authors can use attribute values in any property rather than only as strings in the content property.
Firefox now supports the progress() CSS function.
Firefox now supports the alpha() CSS function. This is a relative color function that can be used to modify just the alpha component of a given color.
Firefox now supports the Promise.allKeyed and Promise.allSettledKeyed methods from the await dictionary proposal, which take an object of promises and resolve to an object of results with the same keys.
Firefox now supports the WebAssembly Compact Import Section proposal, which can reduce the size of WebAssembly modules with many imports.
Firefox now supports the WebAssembly Wide Arithmetic proposal, which adds 128-bit arithmetic instructions for better performance in cryptography and arbitrary-precision math.
Firefox now supports QUIC version 2 (RFC 9369) for HTTP/3 connections, and uses it when the server selects it through compatible version negotiation (RFC 9368).
Support for the non-standard ::-webkit-scrollbar pseudo-element, added in Firefox 153, is now limited to a small list of sites rather than applying universally.
SVG elements now expose the same URL properties as HTML links, including protocol, host, pathname, and search.
SVGGraphicsElement.getBBox() now honors its options argument, so bounding boxes can include stroke, markers, and clipping.
The font-width property is now supported as the standard name for font-stretch.
A dynamic import() that fails due to a network or HTTP error is no longer cached as a failure in the module map, so retrying the same specifier can now succeed.
WebTransport now supports send groups (createSendGroup()) for controlling how bandwidth is shared between streams, writable datagram streams (createWritable()), and exportKeyingMaterial() for deriving keys bound to the TLS session. It also adds a draining promise and subprotocol negotiation through the constructor's protocols option and the protocol property.
Firefox now supports RTCError for data channel failures, RTCSctpTransport.maxChannels, two-byte RTP header extension IDs, and more accurate RTCTransportStats, improving WebRTC interoperability.

Unresolved

For users with vertical tabs enabled, the sidebar may not persist across Firefox restarts. Vertical tabs can be re-enabled after the restart by clicking the sidebar toolbar button. A fix is currently in development. (Bug 2065431)
Some sites may load slowly or fail to load on networks that block UDP. This can affect e.g. corporate networks where a proxy or firewall blocks QUIC as part of TLS inspection. To work around it, set network.http.happy_eyeballs_enabled to false with the Preferences policy. A fix is planned for the 155.0.1 release shipping no later than 8-September. (Bug 2063452)

Community Contributions

With the release of Firefox 155, we are pleased to welcome the developers who contributed their first code change to Firefox in this release, 20 of whom were brand new volunteers! Please join us in thanking each of these diligent and enthusiastic individuals, and take a look at their contributions:

• Artem Bilous: 2056733
• Cameron: 2052559, 2052619
• Colden Cullen: 2060708
• Dan: 1735300
• Fede: 1418178, 2040498, 2046774
• Giulio B: 2048884
• K: 1642578
• Kevin Gosse: 2054036
• Michael F: 2052438
• mikey gough: 2053551
• munawiki: 2055309
• Nathan Leuz: 2053550
• Nirmal Advani: 1804108, 2003174, 2003177, 2003181, 2031618, 2033570, 2046156, 2052618, 2053543, 2053566, 2053568, 2053977, 2058556, 2059866, 2060649
• Priyanshu[:0xanshu]: 1977947
• Richard Van Natta[:rnvannatta]: 2052617
• Ruhollah Majdoddin: 1856645, 2060029, 2061166, 2061348
• Shawn Zivontsis: 2025999
• Simon Rosen: 1875517
• Yuxiao Zhu: 1861034
• Zyphrenn: 2033961, 2059969