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.








  • 方法 DOMTokenList.supports() (en-US) 已经被增加 (bug 1257849).
  • DOMTokenList.replace() 方法已经被增加 (bug 1224186).
  • 第一位的 '?' 字符现在已经被忽视当作为参数在URLSearchParams() 构造器 (bug 1268361).
  • The value returned by URL.origin, HTMLAnchorElement.origin, and HTMLAreaElement.origin (en-US) 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: (bug 1270451).
  • 在预渲染模式, Document.visibilityState 属性现在返回 'prerender' (bug 1069772).
  • Window.isSecureContext 属性已经被实现 (bug 1162772).
  • DOM4 ChildNode.before() (en-US), ChildNode.after() (en-US), ChildNode.replaceWith() (en-US), ParentNode.append() (en-US) and ParentNode.prepend() (en-US) 方法已经被实现 (bug 911477).
  • TouchList.identifiedTouch() (en-US) 方法已经被移除 (bug 1188539).
  • 在默认的情况下, scrollbars Window 特性是开启的当调用 在过去, 这种情况下被强烈推荐开启但却不是默认开启的 (bug 1257887).
  • 增加实验性并且非标准的 HTMLMediaElement.seekToNextFrame() (en-US) 方法, 它允许你which allows seeking frame-by-frame through video content (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 (en-US) 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 (bug 1268852).
  • Support for the third parameter of EventTarget.addEventListener(), either a Boolean or an EventListenerOptions has been added (bug 1266164 and 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 (bug 1272578). See Code values in KeyboardEvent.code 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 (bug 1272599). See Code values in KeyboardEvent.code for a full list of available key codes.
  • The key values "Separator" and "MediaSkip" have been removed, as they were deprecated and unused (bug 1232919).
  • Key values and the corresponding key codes "Hyper" and "Super" have been added to represent these legacy modifier keys (bug 1232919).
  • 两个键值为了多媒体数字小键盘已经被增加: "Key11" and "Key12" (bug 1232919).
  • 下面的心键值被增加为了音频控制按键: "AudioBassBoostToggle", "AudioTrebleDown", and "AudioTrebleUp" (bug 123919).
  • 键值被增加为了麦克风控制按键: "MicrophoneToggle", "MicrophoneVolumeDown", "MicrophoneVolumeUp", and "MicrophoneVolumeMute" (bug 123919).
  • 新的键值被增加为了支持语音识别设备: "SpeechCorrectionList" and "SpeechInputToggle" (bug 1232919).
  • 新的键值已经被增加去支持手机上的特别的按键: "AppSwitch", "Call", "CameraFocus", "EndCall", "GoBack", "GoHome", "HeadsetHook", "LastNumberRedial", "Notification", "MannerMode", and "VoiceDial" (bug 1232919).
  • 这些新的应用程序键值已经被增加: "LaunchContacts" and "LaunchPhone" (bug 1232919).
  • 新的键值被增加去支持电视设备: "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" (bug 1232919).
  • 键值 "MediaSelect" 已经被替换成标准的 "LaunchMediaPlayer" 键值 (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" (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 (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 (bug 1266821). With this dictionary removed, Firefox now simply 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 (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 (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() (en-US) method has been implemented (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 (bug 1271669).

New APIs



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 elminating 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 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 (bug 1251332).



为了增强兼容性对于现在已经存在的内容, Firefox 现在接受一些 webkit 前缀 properties and attributes.

改变对于 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() (en-US) method to the tabs API. This method lets you remove CSS which was previously injected by calling tabs.insertCSS().




See also

Older versions