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Firefox 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.

121.0 Firefox Release

December 19, 2023

Version 121.0, first offered to Release channel users on December 19, 2023


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

  • Firefox now supports Voice Control commands on macOS systems.

  • 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. It is also a known issue that windows are not correctly placed when restoring a previous session on launch.

  • Firefox can now force links to always be underlined. This option can be enabled in the Browsing section of the Firefox Settings menu.

    Screenshot of new Always underline links option

  • The PDF viewer now includes a floating button to simplify deleting drawings, text, and images added in PDFs.

    Screenshot of new floating delete button


  • Various security fixes.

  • Ubuntu Firefox Snap builds did not default to Wayland compositing on some systems as expected when Firefox 121 was first released. This is now fixed and updated builds can be installed with the Ubuntu Software Updater.



  • The Firefox Debugger now includes a new feature: an option to disable the debugger; keyword on the current page. This feature is accessible via a new checkbox in the Breakpoints side panel labeled Pause on debugger statement, located next to the existing Pause on exceptions checkbox. By default, this option is enabled, meaning that the debugger statements are active unless manually disabled.

    Screenshot showing new Pause on debugger statement option

  • As part of our work on improving the accessibility of Firefox Developer Tools, we've unified the focus indicator across the toolbox. The focus indicator is now larger with improved contrast, featuring a white box-shadow for enhanced visibility on blue backgrounds. Adjustments have also been made to ensure clear visibility across shared components and frequently used panels, preventing overlap with element borders or backgrounds.

Web Platform

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

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

  • The balance keyword is now supported for the text-wrap CSS property, which can improve the appearance of short multi-line text blocks such as long headings or captions by harmonizing the line lengths.

  • 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.

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

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