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

No changes







  • Firefox now accepts non-standard Content-Disposition headers with a unquoted filename containing spaces. (bug 1440677)
  • Firefox now supports the HTTP Feature-Policy header web-share directive, which can be used to restrict access to the Web Share API to trusted origins (note that Firefox does not support the Web Share API itself, at time of writing). (bug 1653199)




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

Workers/Service workers

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

WebDriver conformance (Marionette)

  • Now the setWindowRect capability is true by default for all desktop applications (including Thunderbird), and false on Android for GeckoView (bug 1650872).
  • 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 (bug 1661495).
  • In case of failures WebDriver:SwitchToWindow always returns an unified NoSuchWindowError now (bug 1663429).


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

Changes for add-on developers

See also

Older versions