Firefox 81 for developers

This article provides information about the changes in Firefox 81 that will affect developers. Firefox 81 was released on September 22, 2020.

Changes for web developers

Developer Tools



  • Support for the non-standard mozallowfullscreen attribute has been removed from <iframe>. Consider using allow="fullscreen" instead (Firefox bug 1657599).




No changes.




  • The threshold for gamepad joystick activation has been increased. This reduces the chance of inadvertent gamepad activation, both from controllers that send small axis values when they are idle, and very small bumps. (Firefox bug 1539178)

Workers/Service workers

  • Strict MIME type checks are now enforced on worker and shared worker scripts, i.e. scripts targeted by the Worker() and SharedWorker() constructors must now be served with text/javascript (Firefox bug 1569123).

WebDriver conformance (Marionette)

  • The setWindowRect capability is now true by default for all desktop applications (including Thunderbird), and false on Android for GeckoView (Firefox bug 1650872).
  • We've added Fission support for the following commands: WebDriver:SwitchToFrame, WebDriver:SwitchToParentFrame, WebDriver:GetCurrentURL. All Fission-compatible commands are only available when marionette.actors.enabled is set to true.
  • Fixed the broken tracking of browsing contexts after opening a new window (Firefox bug 1661495).
  • In case of failures WebDriver:SwitchToWindow now always returns a unified NoSuchWindowError (Firefox bug 1663429).


  • WebDriver:GetActiveFrame has been removed, because it's not part of the WebDriver specification and is no longer used (Firefox bug 1659502).

Changes for add-on developers

Older versions