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Firefox Nightly
Release Notes

Release Notes tell you what’s new in Firefox. As always, we welcome your feedback. You can also file a bug in Bugzilla or see the system requirements of this release.

121.0a1 Firefox Nightly

October 23, 2023

Version 121.0a1, first offered to Nightly channel users on October 23, 2023

Firefox Nightly gets updated every day and as a consequence, the release notes for the Nightly channel are updated continuously to reflect features that have reached sufficient maturity to benefit from community feedback and bug reports.

Warning: Features listed here may or may not make a final release of Firefox.

In addition to these release notes, you can follow ongoing development on our @FirefoxNightly Twitter account, our Mastodon account as well as read our Nightly Blog.

You can interact with other Firefox Nightly users and give your feedback to Mozilla staff in the Nightly Matrix room on


  • Firefox now prompts Windows users to install the Microsoft AV1 Extension to enable hardware decoding support from about:support if not already installed.

    Bug 1860198
  • Firefox now supports Voice Control commands on macOS systems.

  • Firefox now allows you to force-underline links in websites. This is useful for people with achromatopsia for example.

    Bug 1664278
  • On Linux, Firefox now defaults to the Wayland compositor when available instead of XWayland. This brings support for touchpad & touchscreen gestures, swipe-to-nav, per-monitor DPI settings, better graphics performance, and more.

    Note that due to Wayland protocol limitations, Picture-in-Picture windows require an extra user interaction (generally right-click on the window) or a shell / desktop-environment tweak. See bug 1621261 for related discussion and tracking, this post for a KDE configuration, and this extension for GNOME.

    Bug 1752398

Web Platform

  • Starting with Firefox 120, ULPFE/RED is now on by default for video in Nightly builds. This provides additional protection against packet loss for video during WebRTC calls.

    Bug 1852837
  • The CSS text-indent property now supports the hanging and each-line keywords, offering more convenient paragraph layout for styles such as bibliographies, poetry, etc.

    Bug 784648
  • Firefox now supports text-wrap: balance, which can improve the appearance of short multi-line text blocks such as long headings or captions by harmonizing the line lengths.

    Bug 1860446
  • Lazy loading iframes are now supported (<iframe loading=lazy>). Lazy loading iframes are only loaded when visible, so non-critical iframes can load later when needed to speed up initial page loads, reduce initial network usage, etc.

    Bug 1860729
  • The :has() selector is now supported. This allows authors to match elements that has, or "anchors", at least one element matching its relative selector.

    Bug 1858743
  • Support has been added for tail call elimination in WebAssembly language to improve support for functional languages.

    Bug 1846789

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All Firefox Nightly downloads