# Firefox 155.0b1

- Product: Firefox (https://whatsnew.fyi/product/firefox)
- Vendor: Mozilla
- Date: 2026-08-17
- Version: 155.0b1
- Original notes: https://www.mozilla.org/en-US/firefox/155.0beta/releasenotes/
- Permalink: https://whatsnew.fyi/product/firefox/releases/155.0b1
- Labels: Pre-release

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** — Nintendo Switch Pro and compatible aftermarket controllers now work fully in web games on Windows, with the directional pad and analog sticks correctly mapped
- **added** — Containers can now be reordered in Settings, and the new order is used wherever containers are listed
- **added** — Pages can load faster with Happy Eyeballs v3 trying several ways of connecting to a website at once and using whichever succeeds first
- **added** — The Rules View now offers toggles to emulate the prefers-reduced-motion media feature
- **added** — The JSON Viewer now displays JSON Lines (JSONL/NDJSON) documents, including inline errors for invalid lines
- **added** — 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
- **added** — Firefox now supports the progress() CSS function for computing the position of a value relative to a start and an end
- **added** — SVG <a> elements now expose the same URL properties as HTML links, including protocol, host, pathname, and search
- **added** — 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
- **added** — Firefox now supports the alpha() CSS function for modifying just the alpha component of a given color
- **added** — Firefox now supports the Promise.allKeyed and Promise.allSettledKeyed methods, which take an object of promises and resolve to an object of results with the same keys
- **added** — Firefox now supports QUIC version 2 (RFC 9369) for HTTP/3 connections
- **added** — Firefox now supports the WebAssembly Compact Import Section proposal, which can reduce the size of WebAssembly modules with many imports
- **added** — Firefox now supports the WebAssembly Wide Arithmetic proposal, which adds 128-bit arithmetic instructions for better performance in cryptography and arbitrary-precision math
- **added** — WebTransport adds send groups (createSendGroup()), exportKeyingMaterial(), writable datagram streams (createWritable()), a draining promise, and subprotocol negotiation
- **added** — Firefox 155 improves WebRTC interoperability by adding RTCError for data channel failures, RTCSctpTransport.maxChannels, two-byte RTP header extension IDs, and more accurate RTCTransportStats
- **changed** — Firefox now opens mailto: links only in response to an explicit user action, so pages can no longer launch the default mail client unprompted
- **changed** — PDFs smaller than the paper size can now be enlarged to fill the sheet when printing, by setting the scale above 100%
- **changed** — Firefox now uses firefox-portal-detection.com instead of detectportal.firefox.com to detect captive portals and check network connectivity
- **changed** — Firefox now limits CSS to 75 levels of nested blocks and functions, preventing crashes from extremely deep nesting
- **fixed** — Fixed Firefox preventing Linux systems from going to sleep after a long browsing session

###### New
- 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.
- Pages can load faster, with Happy Eyeballs v3 trying several ways of connecting to a website at once and using whichever succeeds first.
###### Fixed
- Fixed Firefox preventing Linux systems from going to sleep after a long browsing session.
- 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 Windows.
- 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.
- 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.
- Firefox now recovers from the toolbar and tab strip being left unresponsive when the platform fails to end a tab drag session.
- Fixed full screen content being letterboxed on MacBooks with a camera notch, most visibly when watching video.
- On Firefox for Android, autofill prompts now dismiss when focus moves away instead of remaining open until back navigation.
- On Firefox for Android, tabs opened by extensions during private browsing now load in private browsing. Previously they appeared private while their content loaded in regular browsing, which could leave site data behind.
###### Changed
- 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%.
- 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 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.
- On Firefox for Android, long-pressing a toolbar shortcut now offers an Edit shortcut option that goes straight to its setting.
###### Enterprise
- Added the DisableLaunchOnLogin policy to control whether Firefox launches automatically when users log in to Windows. This policy does not apply to Firefox ESR.
- Fixed an issue where locking certain legacy preferences no longer disabled the corresponding controls in Firefox Settings.
###### 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 @ button.
- 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.
- 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.
###### Web Platform
- Firefox now supports the progress() CSS function. This is a math function that can be used in calculations to compute the position of a value relative to a start and an end.
- SVG <a> elements now expose the same URL properties as HTML links, including protocol, host, pathname, and search.
- The CSS attr() function now supports values of any type, so auth

_[Truncated at 4000 characters — full notes: https://www.mozilla.org/en-US/firefox/155.0beta/releasenotes/]_