Firefox 55 for developers
=========================

Firefox 55 was released on August 8, 2017. This article lists key changes that are useful for web developers.

Changes for Web developers
--------------------------

### Developer Tools

* Added filtering of network requests by column values and other properties (Firefox bug 1041895, Firefox bug 1354508, Firefox bug 1354507) and by using regular expressions (Firefox bug 1354495).
* Made it possible to show and hide columns within the Network Monitor (Firefox bug 862855).
* Added remote IP (Firefox bug 1344523), protocol (Firefox bug 1345489), scheme (Firefox bug 1356867), cookies and set cookies columns (Firefox bug 1356869) to Network Monitor.
* The `SourceMap` HTTP header is now supported (prior versions supported the deprecated `X-SourceMap` header, see Firefox bug 1346936).
### HTML

* Elements on which `contenteditable` has been set to `true` now use `<div>` elements to separate different lines of text, to give Firefox parity with other modern browsers (Firefox bug 1297414). See Differences in markup generation for more details.
* Enable `dom.forms.datetime` by default on Nightly (Firefox bug 1366188).
### CSS

* Exposed the `transform-box` property by default (Firefox bug 1208550).
* Implemented the `frames()` timing function (Firefox bug 1248340).
* Implemented the `text-justify` property (Firefox bug 1343512, Firefox bug 276079).
* [css-grid] `fit-content` unexpectedly reserves space for full clamp size in `repeat()` (Firefox bug 1359060).
* The `float` / `clear` logical values — `inline-start` and `inline-end` — which were previously implemented but preffed off in release channels, are now available in all channels by default (Firefox bug 1253919).
* The `layout.css.variables.enabled` preference has been removed completely meaning that the CSS variables feature is enabled all the time and can no longer be disabled (Firefox bug 1312328).
* Implemented the proprietary `-moz-context-properties` property (Firefox bug 1058040).
* Zero (0) angle value without degree unit is not correctly interpreted in `gradient/linear-gradient()` (Firefox bug 1363292).
* The `::cue` pseudo-element is now supported; it matches on text cues presented within a media element (Firefox bug 1318542).
### SVG

* The `<radialGradient>` `fr` attribute has been implemented (Firefox bug 1240275).
### JavaScript

* The `SharedArrayBuffer` and `Atomics` objects are now enabled by default. See A Taste of JavaScript's New Parallel Primitives for an introduction to JavaScript Shared Memory and Atomics.
* The rest operator (`...`) is now supported in object destructuring and the spread operator (`...`) now works in object literals (Stage 3 ECMAScript proposal: Object Rest/Spread Properties, Firefox bug 1339395).
* Async generator methods are now supported (Firefox bug 1353693).
* The `String.prototype.toLocaleLowerCase()` and `String.prototype.toLocaleUpperCase()` methods now support an optional `locale` parameter to specify a language tag for locale-specific case mappings (Firefox bug 1318403).
* The `Intl.Collator` object now supports the `caseFirst` option (Firefox bug 866473).
* The Intl API now uses the browser's default locale instead of the operating system's default locale when no locale setting is provided (Firefox bug 1346674).
* Template call sites objects are now canonicalized per realm, based upon their list of raw strings (Firefox bug 1108941).
* `TypedArray` constructors (like `Int8Array`, `Float32Array`, etc.) have been updated to ES2017. They now use the `ToIndex` operation and allow constructors without arguments, which return zero-length typed arrays (Firefox bug 1317383).
### APIs

#### New APIs

* The Collaborative Scheduling of Background Tasks API (also known as the **Background Tasks API** or the `requestIdleCallback` API) is now enabled by default after being available behind a preference since Firefox 53. This API lets you schedule tasks to be executed when the browser determines that there's free time available before the next repaint occurs, so that your code can make use of that time without causing visible performance lag (Firefox bug 1314959).
* The WebVR 1.1 API is now turned on by default on Windows (and is available on macOS in Nightly). This API exposes virtual reality devices — for example head-mounted displays like the Oculus Rift or HTC Vive — to web apps, enabling developers to translate position and movement information from the display into movement around a 3D scene, and present content into such displays.
* The Intersection Observer API — which provides a way to asynchronously observe changes in the intersection of a target element with an ancestor element or with a top-level document's viewport — has been added (Firefox bug 1321865).

#### DOM

* The `Window` properties `scrollX` and `scrollY` (as well as their aliases `pageXOffset` and `pageYOffset` have been updated to be subpixel precise. Instead of returning an integer, these now return a floating-point value which more accurately describes the scroll position on subpixel-precise displays (Firefox bug 1151421). If need be, you can use `Math.round()` to convert them into integers.
* `MediaQueryList` (and other related features) have been updated to match the latest spec. See Firefox bug 1354441, and also see `MediaQueryList` and `MediaQueryListEvent`.
* Methods of the `DOMTokenList` that modify the list value now automatically trim whitespace and remove duplicate tokens (Firefox bug 869788, also see Trimming of whitespace and removal of duplicates).
* The `HTMLInputElement`'s `maxLength` property can now be dynamically altered with JavaScript after the equivalent HTML has been created (Firefox bug 1352799).
* The `URL()` constructor can no longer accept a `DOMString` as its base (2nd parameter) — it only accepts a `USVString`. It can still use an existing `URL` object for the base, which stringifies itself to the object's `href` attribute (Firefox bug 1368950).

#### DOM events

* The event types supported by the `Document.createEvent()` method have been updated as per the latest DOM spec (Firefox bug 1251198).
* The `MessageEvent.origin` property value is now of type `USVString`, not `DOMString`, and the `MessageEvent.source` property now takes a `MessageEventSource` value (which can be a WindowProxy, `MessagePort`, or `ServiceWorker` object) (Firefox bug 1311324).
* The pinch-to-zoom gesture has now been mapped to the `wheel` event plus the + `Ctrl` key. This mapping was implemented to allow developers to implement simple zooming functionality using the pinch-to-zoom gesture on mobile screens/trackpads (mousewheel + `Ctrl` commonly zooms) (Firefox bug 1052253).

#### Selection API

* The Selection API has been updated so that it has parity with other browsers in terms of how editing hosts are given focus when the selection moves inside them (Firefox bug 1318312). See Behavior of Selection API in terms of editing host focus changes for more details.
* The `Selection` API has been updated to match some recent spec changes (Firefox bug 1359371):
    + The `collapse()` and `extend()` methods' `offset` parameter is now optional.
    + The `collapse()` method's `node` parameter is now nullable.
    + The `containsNode()` method's `partialContainment` parameter is now optional.
    + The `deleteFromDocument()` method has been added.
* Also in the `Selection` API, `Selection.empty()` and `Selection.setPosition()` have been added as aliases of `Selection.removeAllRanges()` and `Selection.collapse()`, for web compat and WebKit/Blink parity reasons (Firefox bug 1359387).
* The `StorageManager.persist()` and `StorageManager.persisted()` methods of the Storage API have been implemented and exposed to `Window` contexts (Firefox bug 1286717).

#### Workers

* Workers and shared workers can now be created with an identifying `name` property. See the `Worker()` and `SharedWorker()` constructors, and the `DedicatedWorkerGlobalScope` and `SharedWorkerGlobalScope` interfaces. (Firefox bug 1364297).
* `setTimeout()` and `setInterval()` are now subject to minimum interval throttling for tracking scripts in background tabs — see Throttling of tracking timeout scripts (Firefox bug 1355311).

#### Service Workers/Push

* Messages sent to service worker contexts (e.g. as the event object of `onmessage` are now represented by `MessageEvent` objects, for consistency with other web messaging features.
* The `PushManager.subscribe()` method now accepts `ArrayBuffer`s and Base64-encoded strings as `applicationServerKey` values (Firefox bug 1337348).

#### Web Audio API

* A non-standard constructor (which accepted a string enum value indicating the purpose for which the context would be used) for `AudioContext` interface was causing errors to be thrown when the `options` parameter was provided. We have removed the non-standard constructor. However, please note that the `options` parameter is not yet supported in Firefox and is currently ignored (Firefox bug 1361475).

#### WebRTC

* `getUserMedia()` now provides a stereo audio stream by default if the source device provides stereo sound; support to specifically request mono input will come in Firefox 56. This only works on desktop at this time; mobile Firefox does not currently support stereo audio input sources (Firefox bug 971528).
* The `getUserMedia()` media capabilities, constraints, and settings `autoGainControl` and `noiseSuppression` now match the spec; formerly they were `moz`-prefixed (Firefox bug 1366415).
* When called with an empty constraints set, `getUserMedia()` was incorrectly returning `NotSupportedError` instead of `TypeError`. This has been fixed (Firefox bug 1349480).
* The following new WebRTC statistics are available: `framesEncoded`, `pliCount`, `nackCount`, and `firCount` (Firefox bug 1348657).
* The `RTCInboundRTPStreamStats` dictionary field formerly called `mozRtt` has been renamed to `roundTripTime` to match the specification; in addition, its behavior has been adjusted to match the standard: it contains a double-precision floating point value which estimates the round-trip time based on the RTCP timestamps in the RTCP Receiver Report, measured in seconds (following the algorithm described in RFC 3550, section 6.4.1). (Firefox bug 1344970). However, please be aware that *this property is moving* to a different dictionary (`RTCRemoteInboundRTPStreamStats`) soon (Firefox bug 1380555).
* The `RTCRTPStreamStats` dictionary now includes the fields `firCount`, `pliCount`, and `nackCount`. These return low-level information that can be used to determine how reliable the connection is (Firefox bug 1348657).
* The `RTCOutboundRTPStreamStats` dictionary now includes the field `framesEncoded`, which reports the number of frames that have been successfully encoded for the stream; with this information, you can compute the frame rate (Firefox bug 1348657).
* On Android, there's now a pref to turn on hardware video encoding to improve video call performance and save on battery. To be enabled by default in Firefox 56 (Firefox bug 1265755).

#### Encrypted Media Extensions API

* Firefox currently allows Encrypted Media Extensions to be used in insecure contexts, despite this not being allowed in the specification. This will be changed in the near future, and starting in Firefox 55, deprecation warnings are output to the web console when this is done. (Firefox bug 1361000).
* Firefox currently doesn't require that at least one `MediaKeySystemCapabilities` object be included in the `suggestedConfigurations` parameter passed into `Navigator.requestMediaKeySystemAccess()`, which the specification does mandate. Starting in Firefox 55, a warning is output to the web console when any audio or video configuration is specified without specifying supported codecs. Soon, failure to include a valid configuration for one or more of audio and video will throw an exception Firefox bug 1368683).

#### WebGL

* The `WEBGL_compressed_texture_s3tc_srgb` extension is now available to WebGL and WebGL2 contexts (Firefox bug 1325113).
### Security

* The `Geolocation` API is now available only to secure contexts (Firefox bug 1072859).
* The `Storage API` is now available only to secure contexts (Firefox bug 1268804).
* The loading of mixed content is now allowed on localhost (Firefox bug 903966).
* Loading of remote JAR files has been disabled again (Firefox bug 1329336).
### Plugins

* Flash content is now "click-to-activate" (Firefox bug 1317856). This was immediately put into effect for all users of Nightly, and 50% of beta users. For Firefox 55 release version, the plan is to activate this for 5% of users 2 weeks after release, 25% of users 4 weeks after release, and 100% of users 6 weeks after release (Firefox bug 1365714).
* Flash and other plugins can no longer be loaded from any URL scheme except for `http://` and `https://` (Firefox bug 1335475).
### Other

* Firefox on Linux can now be made to run in headless mode using the `-headless` flag (see Firefox bug 1356681).
Removals from the web platform
------------------------------

### HTML

* The `xml:base` attribute can no longer be used to set the base URL for paths appearing in the `style` attribute, for example —
 `<div xml:base="https://example.com/" style="background:url(picture.jpg)"></div>` (Firefox bug 1350521).
* The `<style>` element's `scoped` attribute has been hidden behind a pref (`layout.css.scoped-style.enabled`) in content documents in Firefox 55+, as no other browsers support it.
* Support for the obscure `MSThemeCompatible` value of the `<meta>` element's `http-equiv` attribute has been removed from Gecko. No other modern browsers support it, and it was causing compatibility problems (Firefox bug 966240).
### CSS

* The proprietary `:-moz-bound-element` pseudo-class has been removed (Firefox bug 1350147).
* The proprietary `-moz-anchor-decoration` value of `text-decoration-line` has been removed (Firefox bug 1355734).
### APIs

* The `UIEvent.isChar` property has never been supported by any browser but Firefox, and it has never been fully implemented except on macOS. For that reason, it was removed in Firefox 55 to align with other browsers.
* The proprietary Firefox OS Device Storage API has been removed from the platform (Firefox bug 1299500).
* The `aShowDialog` parameter of the non-standard `Window.find()` method (which could be specified to open up a "Find" dialog in the browser) has been removed (Firefox bug 1348409).
* The `HTMLFormElement.requestAutoComplete()` method has been removed (see `HTMLFormElement`) (Firefox bug 1270740).
* The non-standard, Mozilla-specific, WebRTC offer options `mozDontOfferDataChannel` and `mozBundleOnly` have been removed from the `RTCOfferOptions` dictionary and are no longer supported by `RTCPeerConnection.createOffer()` (Firefox bug 1196974).
* Support for the proprietary Firefox OS `Audio Channels API` has been removed from `HTMLMediaElement` and `AudioContext` (Firefox bug 1358061).
### SVG

* The `SVGZoomEvent` and `SVGZoomEvents` interfaces have been removed from the SVG2 spec and Gecko, along with the `onzoom <svg>` attribute (Firefox bug 1314388).
Changes for add-on and Mozilla developers
-----------------------------------------

### WebExtensions

* contextMenus.create()'s command property enables you to open browser action popups, page action popups, and sidebars from the context menu.
* proxy API
* chrome\_settings\_overrides key enables you to override the browser's homepage.
* browser\_style property enables you to have browser-like styling for browser action popups, sidebars, and options pages.
* permissions API
Older versions
--------------

* Firefox 54 for developers
* Firefox 53 for developers
* Firefox 52 for developers
* Firefox 51 for developers
* Firefox 50 for developers
* Firefox 49 for developers
* Firefox 48 for developers
* Firefox 47 for developers
* Firefox 46 for developers
* Firefox 45 for developers
* Firefox 44 for developers
* Firefox 43 for developers
* Firefox 42 for developers
* Firefox 41 for developers
* Firefox 40 for developers
* Firefox 39 for developers
* Firefox 38 for developers
* Firefox 37 for developers
* Firefox 36 for developers
* Firefox 35 for developers
* Firefox 34 for developers
* Firefox 33 for developers
* Firefox 32 for developers
* Firefox 31 for developers
* Firefox 30 for developers
* Firefox 29 for developers
* Firefox 28 for developers
* Firefox 27 for developers
* Firefox 26 for developers
* Firefox 25 for developers
* Firefox 24 for developers