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.
Note: See also And now for … Firefox 84 on Mozilla Hacks.
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 valuesscrollbar-small
(scrollbar-width: thin
is used instead) andscrollbar
(macOS only;scrollbar-horizontal
andscrollbar-vertical
are used instead) (Firefox bug 1673132).
JavaScript
- Custom date/time formats specified as options to the
Intl.DateTimeFormat()
constructor can now includefractionalSecondDigits
— 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 ashttp://localhost/
andhttp://dev.localhost/
— refer to the local host's loopback interface (e.g.http://127.0.0.1
). As a result, resources loaded fromlocalhost
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
andurl
.title
has been removed (Firefox bug 1631464).
Media, WebRTC, and Web Audio
- The
MediaRecorder.start()
method now throws anInvalidModificationError
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
andWebDriver: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 tofalse
(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 tofalse
(Firefox bug 1684827).
Changes for add-on developers
- The
browsingData.remove()
API now supports removing a subset of data types bycookieStoreId
.
Older versions
- Firefox 83 for developers
- Firefox 82 for developers
- Firefox 81 for developers
- Firefox 80 for developers
- Firefox 79 for developers
- Firefox 78 for developers
- Firefox 77 for developers
- Firefox 76 for developers
- Firefox 75 for developers
- Firefox 74 for developers
- Firefox 73 for developers
- Firefox 72 for developers
- Firefox 71 for developers
- Firefox 70 for developers
- Firefox 69 for developers
- Firefox 68 for developers
- Firefox 67 for developers
- Firefox 66 for developers
- Firefox 65 for developers
- Firefox 64 for developers
- Firefox 63 for developers
- Firefox 62 for developers
- Firefox 61 for developers
- Firefox 60 for developers
- Firefox 59 for developers
- Firefox 58 for developers
- Firefox 57 for developers
- Firefox 56 for developers
- Firefox 55 for developers
- Firefox 54 for developers
- Firefox 53 for developers