Firefox 86 for developers

This article provides information about the changes in Firefox 86 that will affect developers. Firefox 86 was released on February 23, 2021.

Note: See also A Fabulous February Firefox — 86! on Mozilla Hacks.

Changes for web developers

Developer Tools


No changes.




  • The Intl.DisplayNames built-in object has been enabled by default. This enables the consistent translation of language, region, and script display names:
    // Get English currency code display names
    let currencyNames = new Intl.DisplayNames(["en"], { type: "currency" });
    // Get currency names
    currencyNames.of("USD"); // "US Dollar"
    currencyNames.of("EUR"); // "Euro"
    For more information see Firefox bug 1654116.



  • is now reset to an empty string if a tab loads a page from a different domain, and restored if the original page is reloaded (e.g. by selecting the "back" button). This prevents an untrusted page from accessing any information that the previous page might have stored in the property (potentially the new page might also modify such data, which might then be read by the original page if it was reloaded). For more information see Firefox bug 1685089.
  • EventTarget.addEventListener() now supports the signal option. This option allows an AbortSignal to be passed to the method. The AbortSignal can later be used to remove the listener by calling abort(). For more information see Firefox bug 1679204.

WebDriver conformance (Marionette)

  • Updated WebDriver:ElementClick to synthesize a mousemove event before the actual click event (Firefox bug 1684002).

Known bugs

  • WebDriver commands following a call to WebDriver:SwitchToFrame can fail with a "no such window" error if the frame's content hasn't yet finished loading (Firefox bug 1691348).
  • After a cross-group 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 (Firefox bug 1690308).


  • Removed support for the deprecated Marionette:ActionChain and Marionette:MultiAction commands (Firefox bug 1683755).

Changes for add-on developers

Older versions