Firefox 77 for developers

This article provides information about the changes in Firefox 77 that will affect developers. Firefox 77 was released on June 2, 2020.

See also the accompanying hacks post — New in Firefox 77: DevTool improvements and web platform updates.

Changes for web developers

Developer tools






WebDriver conformance (Marionette)

  • Certain commands could have caused a hang of Marionette when the currently selected top-level browsing context gets removed (e.g., closing the window by a click) (Firefox bug 1619481).
  • Known issue: Because of changes in Firefox 77 an open user prompt could be closed too early when navigating away from the current page. This problem will be fixed in Firefox 78 (Firefox bug 1631362).

Changes for add-on developers

API changes

Manifest changes


  • Use of the unlimitedStorage permission no longer results in a prompt during extension installation or update. See Requesting the right permissions for more information. (Firefox bug 1630413)
  • Changes related to SameSite cookies mean that when setting host permissions to bypass tracking protection for extension pages hosts must be specified as a full domain or with wildcards. However, for Content scripts tracking protection can only be bypassed for hosts specified with a full domain.

Older versions