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.


  • Firefox now ensures that localhost URLs — such as http://localhost/ and http://dev.localhost/ — refer to the local host's loopback interface (e.g. As a result, resources loaded from localhost are now assumed to have been delivered securely (see Secure contexts), and also will not be treated as mixed content (bug 1220810, bug 1488740).


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