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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.

155.0a1 Firefox Nightly

July 20, 2026

Version 155.0a1, first offered to Nightly channel users on July 20, 2026

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 Bluesky account, our @FirefoxNightly 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 chat.mozilla.org.

New

  • Starting with Firefox 154, We’re testing a simpler tab context menu in Firefox Labs. Turn it on in Settings > Firefox Labs > Streamline tab menu and tell us what you think on Mozilla Connect.

    Bug 2031718

  • Starting with Firefox 154, Firefox Nightly now supports QUIC version 2 (RFC 9369) for HTTP/3 connections. Firefox offers both QUIC v1 and v2 using compatible version negotiation (RFC 9368). The server selects which version is used, so connections upgrade to QUIC v2 when the server chooses it and remain on QUIC v1 otherwise.

    Bug 2040450

Developer

Web Platform

  • Starting with Firefox 154, Firefox Nightly 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.

    Bug 2047437

  • Starting with Firefox 153, Firefox Nightly ships an experimental implementation of the link-parameters property by Tyler Thrailkill.

    Bug 2046153

  • Starting with Firefox 153, Firefox Nightly now supports two new keywords closest-corner and farthest-corner for the radial size in the circle() and ellipse() functions.

    Bug 2045278

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