Firefox 52 for developers

• Completely revamped Responsive Design Mode, including UA selection and network throttling.
• The Animation Inspector now displays timing functions.
• The Page Inspector now includes a CSS grid inspector.
• about:debugging now shows service worker state.
• The Page Inspector includes an easy way to highlight the selected element.
• The Page Inspector displays whitespace-only text nodes.

All devtools bugs fixed between Firefox 51 and Firefox 52.

• The rel="noopener" Link type has been implemented (see Firefox bug 1222516).

New features

• Added :focus-within pseudo-class (Firefox bug 1176997).
• Added support for display:flex/grid and columnset layout inside <button> elements (Firefox bug 984869).
• Implemented interpolation between numeric color and currentcolor (Firefox bug 1299741).
• Implemented flexbox layout for justify-content: space-evenly and align-content: space-evenly (Firefox bug 1235922).
• Added support for subpixel antialiasing in CSS mask / clip-path (Firefox bug 1305259).
• Implemented CSS Text 3 segment break transformation rules (Firefox bug 1081858).
• Basic shape clipping (as applied via the clip-path property) can now be applied to SVG content (Firefox bug 1246741).
• Implemented Flexbox layout for align-self|justify-self: [ first | last ]? baseline (Firefox bug 1221524).
• The touch-action property is now enabled by default on all platforms. (For the full story, see intent to ship mail #1 and intent to ship mail #2.)
• Flexbox align-content handling & single-line-sizing should depend on flex-wrap, not number of lines (Firefox bug 1090031).
• CSS Animations can now be used to animate non-interpolated properties (see Firefox bug 1064937).
• Changed baseline|last-baseline to [ first | last ]? baseline (Firefox bug 1313254).
• The used value for left/right is start for the block-axis (Firefox bug 1221565).
• Stretching flexible tracks with an indefinite containing block length now respects the min/max size(Firefox bug 1309407).
• The initial values of mask-position and mask-repeat have been changed to 0% 0% and repeat, respectively (Firefox bug 1308963).
• There have been a number of changes to CSS <color> values (see Firefox bug 1295456):
  • rgba() and hsla() have now been redefined as aliases of rgb() and hsl(); both accept the same parameter syntax.
  • rgb() and hsl() now accept an optional alpha value, e.g. rgb(255, 0, 0, 0.5).
  • Color functions now accept space-separated parameters rather than commas, e.g. rgb(255 0 0 / 0.5).
  • Alpha values can now be specified as percentages as well as numbers, e.g. rgb(255 0 0 / 50%).
  • The hue component in hsl() colors can now be specified as an angle, as well as a number, e.g. hsl(120deg, 60%, 70%).
• Firefox's implementation of child-indexed pseudo-classes (such as :nth-child, :first-child, and so forth) has been updated to match the CSS selectors level 4 specification: these pseudo-classes now match the appropriate sibling elements rather than the children of their parent element. This allows these pseudo-classes to be used when there is no parent, or the parent is not an Element (Firefox bug 1300374.

CSS Grids

• CSS Grids are implemented.

Changes and removals

• Unprefixed multi-column properties (and added back -moz prefixed versions as aliases, for now) (Firefox bug 1300895).
• Stopped wrapping abspos children of flex container in anonymous flex items(Firefox bug 1269045).
• Implemented grid container baselines (Firefox bug 1151204).
• Removed <flex> min-sizing from the style system (Firefox bug 1305244).
• Remove preference layout.css.masking.enabled (Firefox bug 1308239).
• The proprietary -moz-images-in-menus and -moz-images-in-buttons media types have been removed (see Firefox bug 1302157).
• Removed -moz-use-text-color value from color properties; use currentcolor instead (Firefox bug 1306214).
• [css-grid] 'max-width' set on grid item causes text to overflow (Firefox bug 1330380).

New features

• Support for the async functions has been added. This adds async function, async function expression, and the await keyword (Firefox bug 1185106).
• Implemented ES2017 trailing commas in functions (Firefox bug 1303788).
• Implemented rest parameter destructuring (Firefox bug 1243717).
• The exponentiation operator (**) is now enabled by default (Firefox bug 1291212).
• You can now use IANA time zone names in the timeZone option of date related APIs like DateTimeFormat or Date.toLocaleString() (Firefox bug 837961).

Changes and removals

• Array destructuring now throws a SyntaxError when using destructuring rest with trailing comma (Firefox bug 1041341).
• Duplicate __proto__ properties are now allowed in object destructuring (Firefox bug 1204024).
• Array.prototype.toLocaleString() has been re-implemented to support the Intl API parameters "locales" and "options" (Firefox bug 1130636).
• TypedArray constructors now accept iterables to create new typed arrays (Firefox bug 1232266).
• TypedArray.from(), TypedArray.of(), TypedArray.prototype.filter(), TypedArray.prototype.map(), TypedArray.prototype.slice(), TypedArray.prototype.subarray() now require that their this values are valid Typed Array constructors (Firefox bug 1122396).
• The non-standard ArrayBuffer.slice() method (not ArrayBuffer.prototype.slice()) is deprecated and now presents a warning when used (Firefox bug 1316913).
• Unicode code point escapes can now also be used as identifiers (e.g. "let \u{61} = 123", see Firefox bug 1314037).
• To conform with ES2015, \u2e2f and ⸯ now throw when used as identifier, for details see Firefox bug 917436 and Firefox bug 1197230.

• Support for WebAssembly has been added to Gecko.

• The Selection API has fully shipped, including the new selectstart and selectionchange events (Firefox bug 1309612).
• The property Event.composed is now supported; this Boolean value indicates whether or not the event can bubble through the shadow root into the standard DOM (Firefox bug 1292063).
• Only HTML elements, plus the <svg> and <math> elements, can be put into full-screen mode by calling Element.requestFullscreen() (Firefox bug 1305928).
• Touch events have been reenabled on Windows desktop platforms — see Firefox bug 1244402. (They were disabled in Firefox 24 because they broke a number of major sites; see Firefox bug 888304.)
• The focusin and focusout events are now implemented (Firefox bug 687787).
• The isSecureContext property has been implemented (see Firefox bug 1269052).
• The Web App Manifest install event has been renamed appinstalled (see Window.appinstalled_event, appinstalled) to avoid confusion with the service worker install event (see oninstall). See Firefox bug 1309099 for more details about this update.
• The DataTransfer.types property of the Drag and drop API now returns a frozen array of strings rather than a DOMStringList (see Firefox bug 1298243).
• The loadstart and loadend events are now fired on <img> elements (see Firefox bug 1264769).
• The Notification.requireInteraction of the Notifications API has been implemented (see Firefox bug 862395.)
• The Window.open() method now has a noopener window feature available (see Firefox bug 1267339), which mirrors the functionality of the rel="noopener" Link type.
• The CustomElementRegistry.get() method of the Web Components API has been implemented (see Firefox bug 1275838).
• Pointer Event width and height properties now default to a value of 1 (see Firefox bug 1304315).
• The File and Directory Entries API has been updated to include changes in the latest spec (see Firefox bug 1284987 for the exact details).
• The cancelBubble property, which was defined on UIEvent, is now defined on the Event interface instead. See Firefox bug 1298970 for more details.

Changes and removals

• The Firefox OS APIs that deal with managing phone calls (Contacts, MobileConnection, Icc, etc.) have been removed (Firefox bug 1311206).
• The Firefox OS Identity interface has been removed (Firefox bug 1309030).
• The Firefox OS Voicemail API (MozVoicemail, MozVoicemailEvent, MozVoicemailStatus, Navigator.mozVoicemail) has been removed (Firefox bug 1309723).
• The Firefox OS Cell Broadcast API (MozCellBroadcast, MozCellBroadcastEvent, MozCellBroadcastMessage, Navigator.mozCellBroadcast) has been removed (Firefox bug 1306772).
• The Firefox OS TV broadcast-related APIs have been removed (Firefox bug 1306778).
• The Firefox OS FM Radio API (FMRadio, Navigator.mozFMRadio) has been removed (Firefox bug 1306779).

• The Headers.getAll() method has been removed, and Headers.get() now retrieves all values of the specified header, not just the first one (see Firefox bug 1278275). This is in accordance with the latest Fetch API spec updates.

• The ConstantSourceNode interface has been added; it represents an audio source which always outputs a stream of samples which all have the same value. See Controlling multiple parameters with ConstantSourceNode for an example showing how this can be used to simplify some complex audio flows.

• When an ICE connection is temporarily disrupted, the RTCPeerConnection.iceConnectionState property now gets set to "disconnected"; this indicates a transitory failure that may resolve itself shortly, with the connection returning to the "connected" state afterward (Firefox bug 852665).
• The MediaDevices.devicechange_event event and its corresponding handler, which were implemented but disabled by default on Mac only in Firefox 51, have been implemented on Windows and Linux and are now enabled by default on all platforms.
• The MediaStream.active property is now supported. This read-only Boolean property indicates whether or not at least one track on the stream is currently playing.
• Prior to Firefox 52, the MediaStreamTrack.stop() method could only stop local tracks (that is, tracks obtained through getUserMedia()). Now a variety of tracks can be stopped, including those on a MediaStream associated with a WebRTC connection, Web Audio API stream, or CanvasCaptureMediaStream.
• Previously, changing a TextTrack's mode repeatedly during a single pass through the Firefox event loop would result in multiple change events being delivered to the TextTrackList specified by the parent media element's textTracks property. Now these changes are consolidated into one event (Firefox bug 882674).

• The MediaError objects specified in HTMLMediaElement.error when an error occurs handling an <audio> or <video> element now include a message property, which provides a specific description of the error which occurred. This string offers details particular to this exact error occurrence, offering insight into why things went wrong (Firefox bug 1299072). This field has been included in Firefox nightly builds since Firefox 51, but is now available in all builds, up to and including release.

• The method FileSystemFileEntry.createWriter(), which was added (but always returned an error) in Firefox 50 has been removed (Firefox bug 1315185.
• The proprietary Firefox OS Apps installation/management APIs have been removed from the platform (see Firefox bug 1261019).
• The proprietary Firefox OS Web Telephony API has been removed from the platform (see Firefox bug 1309719).
• The proprietary Firefox OS Web Bluetooth API has been removed from the platform (see Firefox bug 1310020).
• The Battery Status API is now available only to chrome/privileged code (see Firefox bug 1313580).
• ImageBitmapRenderingContext.transferImageBitmap() has been renamed to ImageBitmapRenderingContext.transferFromImageBitmap() (see Firefox bug 1304767).
• The mozDash and mozDashOffset members have been removed from CanvasRenderingContext2D (see Firefox bug 931389).

• The Referrer-Policy header now supports the directives same-origin, strict-origin, and strict-origin-when-cross-origin (Firefox bug 1276836).
• The 'strict-dynamic' source expression is now supported for Content-Security-Policy directives, such as script-src (Firefox bug 1299483).
• Insecure sites (http:) can't set cookies with the "secure" directive anymore as per the Strict Secure Cookies specification (Firefox bug 976073).
• The maximum table size format of the HTTP/2 header compression format HPACK has been increased from 4 KB to 64 KB (Firefox bug 1296280).
• The Large-Allocation header has been added (Firefox bug 1304140).

• SVG documents are now represented using the XMLDocument interface instead of SVGDocument. This is a change made in the SVG 2 specification.

• When login pages (i.e., those containing an <input type="password"> field) are created so that they would be submitted insecurely, Firefox displays an in-context warning message below the password field to warn users (Firefox bug 1319119). Autofill is also disabled on insecure login forms (Firefox bug 1217152). See Insecure passwords for more details.
• Support for SHA-1 SSL certificates has been removed; navigating to a secure page that uses a SHA-1 certificate will now result in an Untrusted Connection error (Firefox bug 1330043).

All NPAPI plugin support except Flash has been dropped. Flash usage is also set to be phased out in the future.

New APIs:

• sessions API
• topSites API
• omnibox API
• runtime.onInstalled and runtime.onStartup events
• asynchronous event listeners in webRequest
• bookmarks.onMoved, bookmarks.onCreated, bookmarks.onChanged events
• _execute_browser_action and _execute_page_action in the commands manifest key
• match_about_blank in the content_scripts manifest key

• nsIDroppedLinkHandler.dropLinks method and nsIDroppedLinkItem interface have been added to handle dropping multiple items (Firefox bug 92737).

• tabbrowser.loadTabs(uris, params) method overload has been added (Firefox bug 92737).
• browser.droppedLinkHandler function signature has been changed (Firefox bug 92737).