# Firefox 155.0

- Product: Firefox (https://whatsnew.fyi/product/firefox)
- Vendor: Mozilla
- Date: 2026-09-01
- Version: 155.0
- Original notes: https://www.mozilla.org/en-US/firefox/155.0/releasenotes/
- Permalink: https://whatsnew.fyi/product/firefox/releases/155.0

What's New is an index, not a publisher: every entry below links to the vendor's own release notes, which are the authoritative source. Entries are labelled where they are hand-curated sample data, pre-releases, or drawn from a secondary source such as a developer blog.

Reuse: the summaries, labels and curation here are © What's New. Quote freely with attribution and a link back; wholesale republication of the corpus is not permitted — terms: https://whatsnew.fyi/terms. The vendors' own release notes remain their publishers'.

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- **added** — Smart Window is now available for users in the USA, Canada, and France
- **added** — Personalized resume conversation starters help users quickly return to topics they've been working on in Assistant
- **added** — Assistant now responds faster with Exa web search source citations
- **added** — Organize Tabs now suggests better groupings of related tabs with more descriptive names
- **added** — Firefox now shows how many trackers it has blocked in the address bar
- **added** — Nintendo Switch Pro and compatible aftermarket controllers now work fully in web games on Windows
- **added** — Containers can now be reordered in Settings
- **added** — Translations feature now supports Marathi and Urdu languages
- **added** — Rules View now offers toggles to emulate the prefers-reduced-motion media feature
- **added** — Debugger in Developer Tools has keyboard shortcut Ctrl+Alt+B (Cmd+Alt+B on macOS) to enable or disable all breakpoints at once
- **added** — JSON Viewer now displays JSON Lines (JSONL/NDJSON) documents with inline errors for invalid lines
- **added** — CSS attr() function now supports values of any type
- **added** — Firefox now supports the progress() CSS function
- **added** — Firefox now supports the alpha() CSS function for modifying the alpha component of colors
- **added** — Firefox now supports Promise.allKeyed and Promise.allSettledKeyed methods
- **added** — Firefox now supports the WebAssembly Compact Import Section proposal
- **added** — Firefox now supports the WebAssembly Wide Arithmetic proposal
- **added** — Firefox now supports QUIC version 2 (RFC 9369) for HTTP/3 connections
- **added** — SVG <a> elements now expose URL properties including protocol, host, pathname, and search
- **fixed** — Fixed text disappearing in Microsoft 365 Word Online and other web apps on Linux when typing with certain keyboard layouts

###### 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.
What is a progressive roll out?
Certain new Firefox features are released gradually. This means some users will see the feature before everyone does. This approach helps to get early feedback to catch bugs and improve behavior quickly, meaning more Firefox users overall have a better experience.
- 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.
What is a progressive roll out?
Certain new Firefox features are released gradually. This means some users will see the feature before everyone does. This approach helps to get early feedback to catch bugs and improve behavior quickly, meaning more Firefox users overall have a better experience.
- 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 emula

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