Firefox 53 for developers

• Avoid scrolling latency on highlighters given by APZ (Firefox bug 1312103).
• Added option to copy the full CSS path of an element (Firefox bug 1323700).
• Devtools support for css-color-4 (Firefox bug 1310681).
• Markup view: add a visual hint between opening and closing tags of a collapsed node (Firefox bug 1323193).

New features

• The mask-* longhand properties (see CSS Masks) are all supported and available by default (see Firefox bug 1251161).
• Added caret-color property (Firefox bug 1063162).
• Implemented the place-items/place-self/place-content shorthands (Firefox bug 1319958).
• Added flow-root value to display property (Firefox bug 1322191).
• -moz-tab-size now accepts <length> values (Firefox bug 943918), and is now animatable (Firefox bug 1308110).
• mask-mode:luminance doesn't work on gradient masks (Firefox bug 1346265).
• [css-grid] FR Unit in grid-template-rows not filling viewport (Firefox bug 1346699).
• flex items aren't sorted according to "order", if they're separated by an abspos sibling (Firefox bug 1345873).

Other changes

• Enable mask longhands on SVG elements (Firefox bug 1319667).
• [css-grid] Fixed: align-self/justify-self:stretch/normal doesn't work on <table> grid items (Firefox bug 1316051).
• Fixed: clip-path: circle() with large reference box and percentage radius does not render correctly (Firefox bug 1324713.
• When applying a text-transform value of uppercase to Greek text, the accent on the disjunctive eta (ή) is no longer removed (see Firefox bug 1322989).
• The availability of the contents value of display was controlled through the layout.css.display-contents.enabled pref. In Firefox 53 this pref has been removed altogether, so the value will always be available and can no longer be disabled (Firefox bug 1295788).

• ECMAScript 2015 semantics for the Function.name properties have been implemented. This includes inferred names on anonymous functions (var foo = function() {}) (Firefox bug 883377).
• ECMAScript 2015 semantics for closing iterators have been implemented. This affects the for...of loop, for example (Firefox bug 1147371).
• The Template Literal Revision proposal that lifts escape sequence restrictions on tagged template literals has been implemented (Firefox bug 1317375).
• The static length property of TypedArray objects was changed from 3 to 0 as per ES2016 (Firefox bug 1317306).
• SharedArrayBuffer can now be used in DataView objects (Firefox bug 1246597).
• In earlier versions of the specification, SharedArrayBuffer objects needed to be explicitly transferred during structured cloning. In the new specification they aren't transferable objects anymore and thus must not be in the transfer list. The new behavior used to present a console warning only, but will now throw an error (Firefox bug 1302037).
• The ArrayBuffer length is now limited to Number.MAX_SAFE_INTEGER (>= 2 ** 53) (Firefox bug 1255128).
• Error and other native error object prototypes like RangeError etc. are now ordinary objects instead of proper Error objects. (In particular, Object.prototype.toString.call(Error.prototype) is now "[object Object]" instead of "[object Error]".) (Firefox bug 1213341).

• CSS Transitions: The transitionstart, transitionrun, and transitioncancel events have been implemented (see Firefox bug 1264125 and Firefox bug 1287983).
• The CompositionEvent constructor has been implemented (see Firefox bug 1002256).
• The MouseEvent.x and MouseEvent.y aliases of MouseEvent.clientX/MouseEvent.clientY have been implemented (see Firefox bug 424390).
• The auxclick event and corresponding event handler have been implemented (see Firefox bug 1304044).
• The transitioncancel event is now fired after a transition is cancelled.

• The pathname and search HTMLHyperLinkElementUtils properties previously returned the wrong parts of the URL. For example, for a URL of http://z.com/x?a=true&b=false, pathname would return "/x?a=true&b=false" and search would return "", rather than "/x" and "?a=true&b=false" respectively. This has now been fixed (Firefox bug 1310483).
• The URLSearchParams() constructor now accepts a string or sequence of strings as an init object (Firefox bug 1330678).
• The Selection.setBaseAndExtent() method of the Selection API is now implemented (see Firefox bug 1321623).
• The "fakepath" addition to file type <input> values has been implemented in Gecko, giving it parity with other browsers (see Firefox bug 1274596).
• Node.getRootNode() has been implemented, replacing the deprecated Node.rootNode property (Firefox bug 1269155).
• Own properties of Plugin and PluginArray objects are no longer enumerable (Firefox bug 1270366).
• Named properties of MimeTypeArray objects are no longer enumerable (Firefox bug 1270364).
• The Permissions API now has a new permission name available — persistent-storage — as used when making a Permissions.query() (see Firefox bug 1270038). This allows an origin to use a persistent box (i.e., persistent storage) for its storage, as per the Storage API.
• The Performance.timeOrigin property has been implemented (Firefox bug 1313420).

• The Network Information API is now available in workers (see Firefox bug 1323172).
• Server-sent events can now be used from workers (see Firefox bug 1267903).
• ExtendableEvent.waitUntil() can now be called asynchronously (see Firefox bug 1263304).

• The WEBGL_compressed_texture_astc WebGL extension has been implemented (Firefox bug 1250077).
• The WEBGL_debug_renderer_info WebGL extension is now enabled by default (Firefox bug 1336645).

General

• Beginning in Firefox 53 for Android, decoding of media is handled out-of-process for improved performance on multi-core systems (Firefox bug 1333323).

Media elements

• The HTMLMediaElement.play() method, used to begin playback of media in any media element, now returns a Promise which is fulfilled when playback begins and is rejected if an error occurs (Firefox bug 1244768).

Web Audio API

• The AudioScheduledSourceNode interface has been added and the AudioBufferSourceNode, ConstantSourceNode, and OscillatorNode interfaces are now based on it (Firefox bug 1324568).
• All the different audio node types have had constructors added to them (Firefox bug 1322883).

WebRTC

• The RTCPeerConnection methods createOffer() and createAnswer() now return a Promise that returns an object conforming to the dictionary RTCSessionDescriptionInit instead of returning an RTCSessionDescription directly. Existing code will continue to work, but new code can be written more simply.
• Similarly, the RTCPeerConnection methods setLocalDescription() and setRemoteDescription() now accept as input an object conforming to the dictionary RTCSessionDescriptionInit. Existing code continues to work, but can be simplified.
• RTCPeerConnection.addIceCandidate() now accepts as input an initialization object. This is compatible with existing code but allows new code to be written slightly more simply when used in tandem with the changes listed above (Firefox bug 1263312).
• DTMF support is now enabled by default using RTCDTMFSender. See Using DTMF with WebRTC for more information on how this works.

• Gecko now has a pref available in about:config to allow users to set their default Referrer-Policy — network.http.referer.userControlPolicy (Firefox bug 1304623). Possible values are:
  • 0 — no-referrer
  • 1 — same-origin
  • 2 — strict-origin-when-cross-origin
  • 3 — no-referrer-when-downgrade (the default)
• Support for Next Protocol Negotiation (NPN) has been removed in favor of Application-Layer Protocol Negotiation (ALPN) — see Firefox bug 1248198.
• The Large-Allocation HTTP header is now available by default, and no longer hidden behind a pref (Firefox bug 1331083).

• Partly implemented SVGGeometryElement interface (Firefox bug 1239100).

• The dom.details_element.enabled pref — which controlled enabling/disabling <details> and <summary> element support in Firefox — has now been removed from about:config. These elements (which were first enabled by default in Firefox 49) can no longer be disabled. See Firefox bug 1271549.
• The mozapp attribute of the <iframe> element /HTMLIFrameElement interface has been removed — this was used to enable a Firefox OS app to be embedded in a moz Browser API <iframe> (Firefox bug 1310845).
• The HTMLIFrameElement.setInputMethodActive() method and InputMethod interface (used to set and manage IMEs on Firefox OS apps) has been removed (Firefox bug 1313169).

• Removed -moz prefixed variant of :dir() pseudo-class (Firefox bug 1270406).
• The -moz prefixed version of text-align-last got removed (Firefox bug 1276808).
• Removed -moz prefixed variant of calc() method (Firefox bug 1331296).
• The proprietary -moz-samplesize media fragment (added to aid in delivery of downsampled images to low memory Firefox OS devices; see Firefox bug 854795) has been removed (Firefox bug 1311246).

• The non-standard ArrayBuffer.slice() method has been removed (but the standardized version ArrayBuffer.prototype.slice() is kept, see Firefox bug 1313112).

• The Wi-Fi information API, Speaker Manager API, and Tethering API, and Settings API have been removed from the platform (see Firefox bug 1313788, Firefox bug 1317853, Firefox bug 1313789, and Firefox bug 1313155 respectively).

• The legacycaller has been removed from the HTMLEmbedElement and HTMLObjectElement interfaces (Firefox bug 909656).

New APIs:

• browsingData
• identity
• contextualIdentities

Enhanced APIs:

• storage.sync
• page_action, browser_action, password, tab context types in contextMenus
• webRequest.onBeforeRequest now supports requestBody
• tabs.insertCSS now supports cssOrigin, enabling you to insert user style sheets.

• The asynchronous [AddonManager APIs]((/en-US/docs/Mozilla/Add-ons/Add-on_Manager/AddonManager) now support Promises as well as callbacks (Firefox bug 987512.