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.

Changes for web developers

Firefox Developer Edition offers a Compatibility panel in the Page Inspector, to show the browsers supported by the CSS properties used on the page. (bug 1625134)
The Debugger has a Settings menu in its toolbar; the only menu option (yet) is Disable JavaScript. (bug 1630957)
The Network Monitor toolbar has an Actions menu for managing log data (bug 1459175):
- Persist Logs
- Import HAR File
- Save All as HAR
- Copy All as HAR
The Request Blocking panel of the Network Monitor has a context menu for enabling, disabling or removing all request block items. (bug 1588076)
If you change the selected source line in the Debugger by clicking a frame in the Call Stack, and then click Step over (F10), the debugger executes until reaching the line following the newly-selected line (disregarding whatever line the debugger originally stopped at). (bug 1630642)
You can set a watchpoint that pauses execution when a value is either read or assigned ("get or set"), as well as just read ("get") or just assigned ("set"). (bug 1580585

Firefox now displays the label value on an <option> element when the element's content is empty. (bug 40545)

Firefox now supports the transform-origin presentation attribute (bug 1581691).

Certain commands could have caused a hang of Marionette when the currently selected top-level browsing context gets removed, eg. closing the window by a click (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 (bug 1631362).

tabs.goBack and tabs.goForward are now supported. (bug 1603796)
browsingData.remove with type serviceWorkers and indexedDB now supports deleting by hostname. (bug 1632990 and bug 1551301)
The tabs.duplicate API now supports duplicateProperties, enabling the position and active status of a duplicated tab to be specified. (bug 1560218)
The permissions API events permissions.onAdded and permissions.onRemoved are now supported. (bug 1444294)
Multiple Content-Security-Policy header changes requested in webRequest.onHeadersReceived are merged. (bug 1462989)

The following permissions are now optional, they can be specified in the optional_permissions manifest key and requested using the permissions API: browsingData (bug 1630417), pkcs11 (bug 1630418), proxy (bug 1548011), and sessions (bug 1630414).

Use of the unlimitedStorage permission no longer results in a prompt during extension installation or update. See Requesting the right permissions for more information. (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.