Firefox 54 release notes for developers

Changes for Web developers

Developer Tools

• The network request summary now includes the amount of data actually transferred ("transferred size"), as does the performance analysis view (Firefox bug 1168376).
• The network request headers view now links to the related documentation on MDN (Firefox bug 1320233).

CSS

• clip-path now supports basic shapes (Firefox bug 1247229).
• Firefox's implementations of CSS Flexbox and CSS alignment now implement updated spec language for interactions between the properties align-items and align-self as well as between justify-items and justify-self (Firefox bug 1340309).
• <input> elements of types checkbox and radio with -moz-appearance: none set on them are now non-replaced elements, for compatibility with other browsers (Firefox bug 605985).
• Previously, an element styled with display: inline-block with a child element of type HTMLInputElement styled with display:block had a wrong baseline (Firefox bug 1330962). This is now fixed.
• When Mozilla introduced dedicated content threads to Firefox (through the Electrolysis or e10s project), support for styling <option> elements was removed temporarily. Starting in Firefox 54, you can apply foreground and background colors to <option> elements again, using the color and background-color attributes. See Firefox bug 910022 for more information. Note that this is still disabled in Linux due to lack of contrast (see Firefox bug 1338283 for progress on this).
• CSS Animations now send the animationcancel event as expected when an animation aborts prematurely (Firefox bug 1302648).
• Transparent colors (i.e., those with an alpha channel of 0) were being serialized to the transparent color keyword in certain situations; this has been fixed so that Firefox follows the spec (as well as other browsers' implementations). See (Firefox bug 1339394 for further information.
• The proprietary :-moz-table-border-nonzero pseudo-class is no longer available to web content; it is now restricted to Firefox's internal UA stylesheet (Firefox bug 1341925).
• [css-grid] Intrinsic content with overflow:auto overlaps in grid (Firefox bug 1348857).
• [css-grid] Transferred min-size contribution of percentage size grid item with an intrinsic ratio (Firefox bug 1349320).

JavaScript

• \b and \B in RegExp with the "u" (Unicode) and "i" (case insensitive) flags now treat U+017F (LATIN SMALL LETTER LONG S) and U+212A (KELVIN SIGN) as word characters (Firefox bug 1338373).
• The DataView constructor now throws a RangeError if the byteOffset parameter is out of Number.MAX_SAFE_INTEGER (>= 2 ** 53) (Firefox bug 1317382).
• The Date.UTC() method has been updated to conform to ECMAScript 2017 when fewer than two arguments are provided (Firefox bug 1050755).
• The Function.prototype.toString() method has been updated to match the latest proposed specification (Firefox bug 1317400).

DOM & HTML DOM

• The URL.toJSON() method has been implemented (Firefox bug 1337702).
• The URLSearchParams() constructor now accepts a record containing strings as an init object (Firefox bug 1331580).
• Values returned in KeyboardEvent.key for printable keys when the control key is also pressed have been corrected on macOS (except when the Command key is pressed) (Firefox bug 1342865).
• The dom.workers.latestJSVersion preference, which was mainly implemented to work around problems using let in workers (due to Firefox bug 855665, which has since been fixed) has been removed (see Firefox bug 1219523).
• The event.timeStamp property now returns a high-resolution monotonic time (DOMHighResTimeStamp) instead of a value representing Unix time in milliseconds.

Web Workers and Service Workers

• WorkerGlobalScope.close is now available on DedicatedWorkerGlobalScope and SharedWorkerGlobalScope instead. This change was made to stop close() being available on service workers, as it isn't supposed to be used there and always throws an exception when called (see Firefox bug 1336043).
• The Window.origin property and the WorkerGlobalScope.origin has been implemented (see Firefox bug 1306170).
• The Client.type property has been implemented (see Firefox bug 1339844).
• Clients.matchAll() now returns Client objects in most recently focused order (see Firefox bug 1266747).
• Some changes have been made to the observed behavior when the Request() constructor is passed an existing Request object instance to make a new instance. The following new behaviors are designed to retain security while making the constructor less likely to throw exceptions:
  • If this object exists on another origin to the constructor call, the Request.referrer is stripped out.
  • If this object has a Request.mode of navigate, the mode value is converted to same-origin.

Audio/Video

General

• 5.1 surround sound playback is now enabled by default on Windows, macOS, and Linux (Firefox bug 1334508, Firefox bug 1321502, and Firefox bug 1323659).

Media Capture and Streams API

• Usage of a MediaStream object as the input parameter to URL.createObjectURL() has been deprecated — the console will now show a warning (see Firefox bug 1334564). You are advised to use HTMLMediaElement.srcObject instead.

Web Audio API

• The method AnalyserNode.getFloatFrequencyData() now correctly represents silent samples in the returned buffer with the value -Infinity (Firefox bug 1336098).
• AudioParam.setValueCurveAtTime() now throws a TypeError exception if any of the specified values aren't finite (Firefox bug 1308437).

Encrypted MediaExtensions API

• The MediaKeySession.keySystem string has been removed from the specification, and as such we've taken it out of Firefox 54 (Firefox bug 1335555).
• Support has been added for the VP9 codec in encrypted streams using Clear Key and Widevine (Firefox bug 1338064).
• Previously, MSE was only allowed to use WebM/VP8 video if the system was considered "fast enough." Now playback of VP8-encoded webm/video media is always supported, regardless of system performance.

WebRTC

• TCP ICE candidate support, originally added in Firefox 41, is now enabled by default. This allows the ICE layer to consider candidates that use TCP rather than the preferred UDP for transmission. This can be useful in environments in which UDP is blocked (Firefox bug 1176382). This blog post explains the feature in more details.

Removals from the web platform

CSS

• Removed the -moz prefixed versions of isolate, isolate-override, and plaintext values for the unicode-bidi property (Firefox bug 1333675).

HTTP

• HTTP/1 Pipelining support has been removed in Firefox 54. Maintaining it as we make the move into a new world full of HTTP/2 and other substantial, standardized improvements to networking performance is not worthwhile given pipelining's compatibility and performance issues. The network.http.pipelining preference (as well as the other preferences that start with "network.http.pipelining") is now ignored. See Firefox bug 1340655 for further information.