Firefox 84 release notes for developers

Changes for web developers

Developer Tools

• 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 (Firefox bug 1654956).

HTML

No changes.

CSS

• We've added support for complex selectors to the :not pseudo-class (Firefox 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) (Firefox bug 1673132).

JavaScript

• 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 (Firefox bug 1645107).

HTTP

No changes.

Security

• 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 (Firefox bug 1220810, Firefox bug 1488740).

APIs

• We've added support for the PerformancePaintTiming interface of the Paint Timing API (Firefox bug 1518999).
• The Navigator.registerProtocolHandler() method now only accepts two parameters: scheme and url. title has been removed (Firefox 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 (Firefox bug 1581139).

Removals

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

WebAssembly

No changes.

WebDriver conformance (Marionette)

• Added chrome scope support for WebDriver:PerformActions and WebDriver:ReleaseActions (Firefox 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 (Firefox bug 1669169).
• Fixed WebDriver:SwitchToWindow to always switch back to the top-level browsing context (Firefox bug 1305822).
• Improved browsing context checks for WebDriver:SwitchToParentFrame (Firefox bug 1671622).
• Fixed a hang for WebDriver:Back encountered when the currently-selected <iframe> gets unloaded (Firefox 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 (Firefox bug 1684827).

Changes for add-on developers

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