Firefox 52 for developers

Firefox 52 was released on March 7, 2017. This article lists key changes that are useful not only for web developers, but also Firefox and Gecko developers as well as add-on developers.

Changes for Web developers

Developer Tools



New features

  • Added :focus-within pseudo-class (bug 1176997).
  • Added support for display:flex/grid and columnset layout inside <button> elements (bug 984869).
  • Implemented interpolation between numeric color and currentcolor (bug 1299741).
  • Implemented flexbox layout for justify-content: space-evenly and align-content: space-evenly (bug 1235922).
  • Added support for subpixel antialiasing in CSS mask / clip-path (bug 1305259).
  • Implemented CSS Text 3 segment break transformation rules (bug 1081858).
  • Basic shape clipping (as applied via the clip-path property) can now be applied to SVG content (bug 1246741).
  • Implemented Flexbox layout for align-self|justify-self: [ first | last ]? baseline (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 (bug 1090031).
  • CSS Animations can now be used to animate non-interpolated properties (see bug 1064937).
  • Changed baseline|last-baseline to [ first | last ]? baseline (bug 1313254).
  • The used value for left/right is start for the block-axis (bug 1221565).
  • Stretching flexible tracks with an indefinite containing block length now respects the min/max size(bug 1309407).
  • The initial values of mask-position and mask-repeat have been changed to 0% 0% and repeat, respectively (bug 1308963).
  • There have been a number of changes to CSS <color> values (see 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 (bug 1300374.

CSS Grids

Changes and removals

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


New features

Changes and removals



Changes and removals

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

Service Workers and Fetch

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

Web Audio API


  • 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 (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 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 (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 (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.

Other APIs



  • 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 (bug 1319119). Autofill is also disabled on insecure login forms (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 (bug 1330043).


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

Changes for add-on and Mozilla developers



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


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

Older versions