Firefox 84 for developers

This article provides information about the changes in Firefox 84 that will affect developers. Firefox 84 was released on December 15, 2020.

• The Firefox Accessibility Inspector now supports displaying the keyboard tab order on a web page. This provides a better high-level overview of how the page will be navigated using the keyboard than tabbing through the links (bug 1654956).

No changes.

• We've added support for complex selectors to the :not pseudo-class (bug 933562).

Removals

• We've removed the proprietary -moz-default-appearance property values scrollbar-small (scrollbar-width: thin is used instead) and scrollbar (macOS only; scrollbar-horizontal and scrollbar-vertical are used instead) (bug 1673132).

• Custom date/time formats specified as options to the Intl.DateTimeFormat() constructor can now include fractionalSecondDigits — the number of digits used to represent fractions of a second (bug 1645107).

No changes.

• Firefox now ensures that localhost URLs — such as http://localhost/ and http://dev.localhost/ — refer to the local host's loopback interface (e.g. http://127.0.0.1). As a result, resources loaded from localhost are now assumed to have been delivered securely (see Secure contexts), and also will not be treated as mixed content (bug 1220810, bug 1488740).

• We've added support for the PerformancePaintTiming interface of the Paint Timing API (bug 1518999).
• The Navigator.registerProtocolHandler() method now only accepts two parameters: scheme and url. title has been removed (bug 1631464).

Media, WebRTC, and Web Audio

• The MediaRecorder.start() method now throws an InvalidModificationError if the number of tracks on the stream being recorded has changed (bug 1581139).

Removals

• The application cache has been removed — developers should use the Service Worker API instead (bug 1619673).

No changes.

• Added chrome scope support for WebDriver:PerformActions and WebDriver:ReleaseActions (bug 1365886).
• The new Fission-compatible API has been enabled by default now. To revert to the former API the marionette.actors.enabled preference has to be set to false (bug 1669169).
• Fixed WebDriver:SwitchToWindow to always switch back to the top-level browsing context (bug 1305822).
• Improved browsing context checks for WebDriver:SwitchToParentFrame (bug 1671622).
• Fixed a hang for WebDriver:Back encountered when the currently-selected <iframe> gets unloaded (bug 1672758).

Known bugs

• After 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 (bug 1684827).

• The browsingData.remove() API now supports removing a subset of data types by cookieStoreId.