Firefox 84 for developers

This article provides information about the changes in Firefox 84 that will affect developers. Firefox 84 was released on December 15, 2020.

Note: See also And now for … Firefox 84 on Mozilla Hacks.

Changes for web developers

Developer Tools


No changes.


  • We've added support for complex selectors to the :not pseudo-class (bug 933562).


  • We've removed the proprietary -moz-default-appearance property values scrollbar-small (scrollbar-width: thin is used instead) and scrollbar (macOS only; scrollbar-horizontal and scrollbar-vertical are used instead) (bug 1673132).



No changes.



Media, WebRTC, and Web Audio



No changes.

WebDriver conformance (Marionette)

  • Added chrome scope support for WebDriver:PerformActions and WebDriver:ReleaseActions (bug 1365886).
  • The new Fission-compatible API has been enabled by default now. To revert to the former API the marionette.actors.enabled preference has to be set to false (bug 1669169).
  • Fixed WebDriver:SwitchToWindow to always switch back to the top-level browsing context (bug 1305822).
  • Improved browsing context checks for WebDriver:SwitchToParentFrame (bug 1671622).
  • Fixed a hang for WebDriver:Back encountered when the currently-selected <iframe> gets unloaded (bug 1672758).

Known bugs

  • After page navigation, accessing a previously-retrieved element might not always raise a "stale element" error, and can also lead to a "no such element" error. To prevent this, set the marionette.actors.enabled preference to false (bug 1684827).

Changes for add-on developers

Older versions