Firefox 79 for developers

This article provides information about the changes in Firefox 79 that will affect developers. Firefox 79 was released on July 28, 2020.

Changes for web developers

Network messages with response codes in the 400-499 and 500-599 ranges are now considered errors, and are displayed even if Response or XHR filters are disabled. (Firefox bug 1635460)
Network messages for requests that are blocked (by the browser or an extension) are now styled with a "prohibited" icon in the Console. (Firefox bug 1629875)

"Blackbox" a source file is now called "ignore" a source file. (Firefox bug 1642811)
Inline preview is now available on exceptions. (Firefox bug 1581708)
Items in the Watch Expressions and Scopes sections now have tooltips on hover, showing their values (Firefox bug 1631545)
In the Call Stack section, there is now a context menu option to Restart Frame, to execute the current stack frame from its beginning. (Firefox bug 1594467)

The new Application panel is now available, which initially provides inspection and debugging support for service workers and web app manifests.
The Messages tab of the Network Monitor has been merged with the Responses tab. (Firefox bug 1636421)
The Accessibility Inspector is automatically turned on when you access its tab; you no longer need to explicitly enable it. (Firefox bug 1602075)
In Responsive Design Mode, when touch simulation is enabled, mouse-drag events are now interpreted as touch-drag or swipe events. (Firefox bug 1621781)
When remote debugging, the URL bar now has Back and Forward buttons to help with navigation on the remote browser. (Firefox bug 1639425)

The <iframe> element's sandbox attribute now supports the allow-top-navigation-by-user-activation token (Firefox bug 1359867).
Setting target="_blank" on <a> and <area> elements implicitly provides the same behavior as also setting rel="noopener" (Firefox bug 1522083).

External style sheets are now cached per document group (Firefox bug 1599160). Firefox will minimize retrieval and revalidation of cached style sheets when navigating pages on the same origin. A simple reload (for example, F5) will not revalidate the cached CSS files. To load current versions of the style sheets, reload the page bypassing the cache (Cmd/Ctrl + F5).

The prefers-color-scheme media feature's no-preference value has been removed from the media queries spec, and from Firefox (Firefox bug 1643656).

SharedArrayBuffer has been re-enabled in a post-Spectre-safe manner. It is available to cross-origin isolated sites (Firefox bug 1619649).
- To cross-origin isolate your site, you need to set the new Cross-Origin-Embedder-Policy (COEP) and Cross-Origin-Opener-Policy (COOP) headers.
Promise.any() is now available (Firefox bug 1599769).
WeakRef objects have been implemented (Firefox bug 1639246).
Logical assignment operators are now supported (Firefox bug 1639591)
Atomics objects now also work with non-shared memory (Firefox bug 1630706).
The Intl.DateTimeFormat() constructor now supports the dateStyle and timeStyle options (Firefox bug 1557718).
The Intl.NumberFormat() constructor now supports more numbering systems (Firefox bug 1413504).

Cross-origin isolation has been implemented using the new Cross-Origin-Embedder-Policy (COEP) and Cross-Origin-Opener-Policy (COOP) headers. This allows you to access certain features such as SharedArrayBuffer objects and unthrottled timers in Performance.now().

The FileReader interface's loadstart event is now dispatched asynchronously, as per the spec (Firefox bug 1502403).
CanvasPattern.setTransform() now supports a DOMMatrix object as an input parameter, as well as an SVGMatrix object (Firefox bug 1565997).

Firefox now supports remote timestamps on statistics records whose RTCStats.type is remote-outbound-rtp. The RTCRemoteOutboundRtpStreamStats dictionary which is used to provide these statistics now includes the remoteTimestamp property, which states the timestamp on the remote peer at which the statistics were collected or generated (Firefox bug 1615191).

A number of internal Gecko events — including DOMWindowClose — which were accidentally exposed to the web, are now internal-only as intended (Firefox bug 1557407).