Firefox 49 for developers

To test the latest developer features of Firefox, install Firefox Developer Edition Firefox 49 was released on September 20, 2016. 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






  • The method DOMTokenList.supports() has been added (Firefox bug 1257849).
  • The DOMTokenList.replace() method has been added (Firefox bug 1224186).
  • Leading '?' characters are now ignored in the parameter of the URLSearchParams() constructor (Firefox bug 1268361).
  • The value returned by URL.origin, HTMLAnchorElement.origin, and HTMLAreaElement.origin for URL using the blob: scheme is no longer incorrectly null but is instead the origin of the URL formed by removing the leading blob: (Firefox bug 1270451).
  • In prerendering mode, the Document.visibilityState property now returns 'prerender' (Firefox bug 1069772).
  • The Window.isSecureContext property has been implemented (Firefox bug 1162772).
  • The DOM4 Element.before, Element.after, Element.replaceWith, Element.append and Element.prepend methods have been implemented (Firefox bug 911477).
  • The TouchList.identifiedTouch() method has been removed (Firefox bug 1188539).
  • By default, the scrollbars Window feature is enabled when calling In the past, while it was strongly recommended to enable it, it wasn't the default (Firefox bug 1257887).
  • Added the experimental and non-standard HTMLMediaElement.seekToNextFrame() method, which allows seeking frame-by-frame through video content (Firefox bug 1235301). While you're encouraged to experiment with this method to help us understand how useful it is, do not use it in production code!
  • The HTMLLabelElement.form property now returns the form with which the label's control is associated, if there is a control (and if that control is associated with a form). Previously, labels were directly associated with forms using this property (Firefox bug 1268852).
  • Support for the third parameter of EventTarget.addEventListener(), either a Boolean or an EventListenerOptions has been added (Firefox bug 1266164 and Firefox bug 1266066).
  • The audio volume related values for KeyboardEvent.key have been renamed. "VolumeDown" is now "AudioVolumeDown", "VolumeUp" is now "AudioVolumeUp", and "VolumeMute" is now "AudioVolumeMute". This brings Firefox into alignment with the latest draft of the UI Events specification (Firefox bug 1272578). See Code values for keyboard events for a full list of available key codes.
  • The keys previously referred to as "MozHomeScreen", "MozCameraFocusAdjust", and "MozPhoneCall" now have official names in the UI Events specification: "GoHome", "CameraFocus", and "Call". Firefox 49 has been updated to use the new names (Firefox bug 1272599). See Code values for keyboard events for a full list of available key codes.
  • The key values "Separator" and "MediaSkip" have been removed, as they were deprecated and unused (Firefox bug 1232919).
  • Key values and the corresponding key codes "Hyper" and "Super" have been added to represent these legacy modifier keys (Firefox bug 1232919).
  • Two key values for multimedia numeric keypad keys have been added: "Key11" and "Key12" (Firefox bug 1232919).
  • The following new key values have been added for audio control keys: "AudioBassBoostToggle", "AudioTrebleDown", and "AudioTrebleUp" (Firefox bug 123919).
  • Key values have been added for these microphone control keys: MicrophoneToggle, MicrophoneVolumeDown, MicrophoneVolumeUp, and MicrophoneVolumeMute (Firefox bug 123919).
  • New key values have been added to support speech recognition devices: SpeechCorrectionList and SpeechInputToggle (Firefox bug 1232919).
  • New key values have been added to support special buttons on phones: AppSwitch, Call, CameraFocus, EndCall, GoBack, GoHome, HeadsetHook, LastNumberRedial, Notification, MannerMode, and VoiceDial (Firefox bug 1232919).
  • These new application key values have been added: LaunchContacts and LaunchPhone (Firefox bug 1232919).
  • New key values have been added to support television devices: TV3DMode, TVAntennaCable, TVAudioDescription, TVAudioDescriptionMixDown, TVAudioDescriptionMixUp, TVContentsMenu, TVDataService, TVInput, TVInputComponent1, TVInputComponent2, TVInputComposite1, TVInputComposite2, TVInputHDMI1, TVInputHDMI2, TVInputHDMI3, TVInputHDMI4, TVInputVGA1, TVMediaContext, TVNetwork, TVNumberEntry, TVRadioService, TVSatellite, TVSatelliteBS, TVSatelliteCS, TVSatelliteToggle, TVTerrestrialAnalog, TVTerrestrialDigital, TVTimer, and DVR (Firefox bug 1232919).
  • The key value MediaSelect has been replaced with the standard LaunchMediaPlayer key value (Firefox bug 1272592).
  • Additional media player key values have been added as well. These are MediaAudioTrack, MediaSkipBackward, MediaSkipForward, MediaStepBackward, MediaStepForward, MediaTopMenu, NavigateIn, NavigateNext, NavigateOut, and NavigatePrevious (Firefox bug 1232919).




  • The Fetch API's Response object now implements the redirected property, which indicates whether or not the response is for a request which was redirected. Please review the security related notes in the documentation before using this property (Firefox bug 1243792).
  • In the Permissions API, Firefox no longer supports the 'push' PermissionDescriptor dictionary type (referred to in the spec as PushPermissionDescriptor); this is because Firefox relies on a quota system for controlling the userVisibleOnly status instead, and was throwing an error when it encountered a PushPermissionDescriptor instance (Firefox bug 1266821). With this dictionary removed, Firefox now ignores it.

Media Streams

  • In the past, it was possible for a call to MediaDevices.getUserMedia() which requests both audio and video to succeed in cases where the user has only one of the two types of hardware available. This has been fixed (Firefox bug 802326).
  • In prior versions of Firefox, it was possible for a call to MediaDevices.getUserMedia() which requests both audio and video to succeed even though the user denied access to one but not both of the matching devices. This has been fixed (Firefox bug 802326). This involves minor user interface changes as well, to remove the options to choose "No audio" or "No video" when the user is prompted for permissions.
  • The MediaStream.getTrackById() method has been implemented (Firefox bug 1208390).


  • The RTCPeerConnection.addTrack() method has been updated to allow tracks which are not components of the specified streams to be added to the connection. Instead, the streams are used to group tracks on the receiving end of the connection (Firefox bug 1271669).

New APIs



No change.


  • Removed support for the deprecated <altGlyph>, <altGlyphDef> and <altGlyphItem> elements (Firefox bug 1260032).


No change.

Plugins and Flash

Beginning in Firefox 49, Firefox, by default, blocks certain kinds of Flash content that aren't necessary for sites to function well. This behavior, controlled by the preference browser.safebrowsing.blockedURIs.enabled, helps to improve the performance of sites and Firefox in general without having significant impact on site usability. It also helps improve stability of the browsing experience by eliminating a major cause of crashes. The blocked Flash modules include several used just for fingerprinting purposes, as well as a number of "supercookie" modules, and in the future may be expanded to include more types of blocked modules. See Firefox bug 1275591 for details.

This marks the next step in the journey toward a plugin-free future. HTML is very close to the point where plugins will no longer be needed to get the job done.



  • The Proxy Auto-Configuration (PAC) implementation has been updated. Now weekdayRange, dateRange, and timeRange support "reversed ranges", for example, weekdayRange("SAT", "MON") will evaluate true if the current day is Saturday, Sunday, or Monday (Firefox bug 1251332).



In order to improve compatibility with existing content, Firefox now accepts some WebKit prefixed properties and attributes.

  • The following properties now also work prefixed with -webkit:
    • -webkit-align-items
    • -webkit-align-content
    • -webkit-align-self
    • -webkit-animation
    • -webkit-animation-delay
    • -webkit-animation-direction
    • -webkit-animation-duration
    • -webkit-animation-fill-mode
    • -webkit-animation-iteration-count
    • -webkit-animation-name
    • -webkit-animation-play-state
    • -webkit-animation-timing-function
    • -webkit-backface-visibility
    • -webkit-background-clip
    • -webkit-background-origin
    • -webkit-background-size
    • -webkit-border-bottom-left-radius
    • -webkit-border-bottom-right-radius
    • -webkit-border-image
    • -webkit-border-top-left-radius
    • -webkit-border-top-right-radius
    • -webkit-border-radius
    • -webkit-box-shadow
    • -webkit-filter
    • -webkit-flex
    • -webkit-flex-basis
    • -webkit-flex-direction
    • -webkit-flex-flow
    • -webkit-flex-grow
    • -webkit-flex-shrink
    • -webkit-flex-wrap
    • -webkit-justify-content
    • -webkit-order
    • -webkit-perspective
    • -webkit-perspective-origin
    • -webkit-text-size-adjust
    • -webkit-transform
    • -webkit-transform-origin
    • -webkit-transform-style
    • -webkit-transition
    • -webkit-transition-delay
    • -webkit-transition-duration
    • -webkit-transition-property
    • -webkit-transition-timing-function
    • -webkit-user-select
  • The following properties map to the equivalent prefixed property:
    • -webkit-box-flex
    • -webkit-box-ordinal-group
    • -webkit-box-orient
    • -webkit-box-align
    • -webkit-box-pack
  • For <image> values:
    • The following functions map to their unprefixed equivalents: -webkit-linear-gradient(), -webkit-radial-gradient(), -webkit-repeating-linear-gradient(), and -webkit-repeating-radial-gradient().
    • The outdated -webkit-gradient is supported (and translated to a regular gradient)
  • The following display values are translated:
    • -webkit-box to -moz-box
    • -webkit-flex to flex
    • -webkit-inline-box to inline-flex
    • -webkit-inline-flex to -moz-inline-flex
  • The following properties are supported (and don't map to any unprefixed equivalent):
  • The WebKitCSSMatrix interface is an alias of DOMMatrix
  • The following media query features have been implemented:
    • -webkit-min-device-pixel-ratio as an alias of min-resolution with the same value (in dppx), though this feature is disabled by default (behind about:config pref layout.css.prefixes.device-pixel-ratio-webkit)
    • -webkit-max-device-pixel-ratio as an alias of max-resolution of the same value (in dppx); this feature is also disabled by default, behind the same about:config pref.
    • -webkit-transform-3d always matching, indicating 3d transform support.

Changes for add-on and Mozilla developers


  • Support for the history has been added. This provides access to the browser history, with methods available for searching the history, getting information about previously-visited pages, and adding and removing history entries.
  • Added the tabs.removeCSS() method to the tabs API. This method lets you remove CSS which was previously injected by calling tabs.insertCSS().



No change.

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