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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.

91.0a1 Firefox Nightly

May 31, 2021

Version 91.0a1, first offered to Nightly channel users on May 31, 2021

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 as well as read our Nightly Blog.


  • Firefox now supports logging into Microsoft, work, and school accounts using Windows single sign-on. Learn more

    Bug 1716360
  • The simplify page when printing feature is back! When printing, under More settings > Format select the Simplified option when available to get a clutter-free page. Learn more

    Bug 1666247
  • We've added a new locale: Scots (sco)

    Bug 1716987
  • The address bar now provides Switch to Tab results also in Private Browsing windows.

    Bug 1686951
  • Firefox now automatically enables High Contrast Mode when "Increase Contrast" is checked on MacOS

    Bug 1713015

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