Firefox 151 release notes for enterprise admins

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Firefox 151

Released 19 May 2026.

These changes apply to Firefox 151 and Firefox ESR 140.11.0 unless explicitly stated. Firefox ESR 140 is the current ESR, and enterprise changes are no longer backported to Firefox ESR 128.

Firefox 153 will be the next ESR. As that transition approaches, fewer non-security fixes and policy changes will be backported to Firefox ESR 140. If you use the ESR, we recommend testing with Firefox Beta or Nightly to identify issues before Firefox ESR 153 ships.

New in Firefox 151

• DefaultSerialGuardSetting: Control use of the Web Serial API. Does not apply to the ESR.
• XSLTEnabled: Enable or disable support for the XSLTProcessor JavaScript API and the XSLT processing instruction. Does not apply to the ESR.

Changes

• Policy documentation has officially moved to https://firefox-admin-docs.mozilla.org/. Update any existing bookmarks.
• Preferences: Allow setting preferences under the devtools. and sidebar. branches.
• ExtensionSettings: Support overriding update URLs. Firefox 151 only.

Fixes

• DisableRemoteImprovements: The policy was not reflected correctly in preferences. Firefox 151 only.
• Sites blocked by enterprise policy did not display the correct error message. Firefox 151 only.