Firefox 8 for developers

Firefox 8 was released on November 8, 2011. This article provides information both for web developers and for add-on and Mozilla project developers to help take full advantage of the features of this release.

Changes for web developers


  • The HTMLImageElement crossOrigin property has been added and the crossorigin attribute has been added to the <img> element. (see Firefox bug 664299)
  • The HTMLSelectElement.add() method now supports either an item or index of an item that the new item should be inserted before. Previously it only supported an item. (see Firefox bug 666200)
  • The HTMLIsIndexElement constructor has been removed. No elements have implemented this interface since before Firefox 4.
  • The HTML5 "context menu" feature (contextmenu attribute), which lets you add custom element specific items to native context menu, is now supported (the implementation is still experimental awaiting changes in the specification; see Firefox bug 617528).
  • Support for the HTMLElement.accessKeyLabel attribute has been added to all elements.
  • The <input> and <textarea> elements now support the selectionDirection attribute, and their setSelectionRange() methods have been updated to support optionally specifying a direction.
  • Most elements now get a focus ring drawn around them if they've been made focusable by using the tabindex attribute and the user then focuses the element.
  • In a set of nested <label> elements click events do no longer trigger multiple <label> elements, which caused Firefox to stop responding in the past (see Firefox bug 646157).


  • The insertAdjacentHTML method has been implemented.
  • BlobBuilder now has a getFile() method that returns the content of the blob as a file.
  • The FileReaderSync interface (part of the FileAPI) has been implemented.
  • Event handling in nested <label>s has been fixed.
  • You can now use window.postMessage() to pass File and FileList objects between windows.
  • When editing contenteditable areas, exiting a heading by pressing return, or exiting list editing mode by pressing return twice, now returns to paragraph entry mode (that is, paragraphs inside <p> blocks) instead of separating lines by <br> elements.
  • Fixed a bug that prevents justification from taking effect properly when applied to the first line in a contenteditable area.
  • Fixed a bug that caused pressing delete or backspace at the beginning of a contenteditable area to affect the previous contenteditable block if one is present.
  • Document.getSelection() now returns the same Selection object as window.getSelection(), instead of stringifying it.
  • The HTML5 selectionDirection property makes it possible to define the direction of the selection in an editable text.
  • HTMLMediaElement now have a seekable property that return a TimeRanges object.
  • The HTMLMediaElement.preload attribute now correctly reflects as an enumerated value.
  • crossOrigin property defaults to "Anonymous" when an invalid value is used.
  • navigator.cookieEnabled now returns correct information when the default cookie setting is overridden on a per-site basis.


  • RegExp.exec() and RegExp.test() called with no arguments now match against the string "undefined".
  • and String.match() called with no arguments or undefined now match against the empty string and thus match every string.
  • Support for watchlists has been implemented with the new (non-standard) watch() and unwatch() methods.


  • <resolution> now accepts <number>, not just <integer> values as per the specification.
  • Hyphenation rules have been added for many new languages when using hyphens.
  • Handling of background-size has been revised to more closely match the specification.
  • In the past, text-decoration in quirks mode had line thickness and position adjusted on descendant text to match the descendant. Now standards mode and quirks mode rendering are more similar.
  • Horizontal positioning for elements has been brought more in line with the specification in many cases. Documentation for this is forthcoming, but for now, see Firefox bug 682780, comment 23 for details.
  • SVG images are now scaled properly when used as background images.


  • Double quotes are no longer accepted as a delimiter for RFC 2231 or RFC 5987 encoding, as per those RFCs.
  • MIME header field parser (Content-Disposition) now requires "=" in parameters.
  • Scripts are no longer downloaded when JavaScript is disabled.
  • SSL 2.0 is no longer supported.


  • The WebSocket object's send() method no longer incorrectly returns a Boolean value.
  • The WebSocket object's close() method now matches the current draft of the standard, and close events now properly use the CloseEvent interface.
  • The WebSocket object's extensions attribute is now supported.
  • The WebSocket constructor now supports an array of protocols as well as a single protocol string.
  • Mixed content is not allowed with WebSockets; that is, you can no longer open a connection to a non-secure WebSocket server from secure content.
  • Connection errors with WebSockets now trigger the onerror handler.
  • WebSocket API has been updated to the latest draft of the specification (see Firefox bug 674890, Firefox bug 674527, and Firefox bug 674716).
  • The deflate-stream extension to WebSockets has been disabled; it has been deprecated, and was breaking compatibility with some sites.


  • Cross-domain textures can now be allowed with CORS approval.
  • Cross-process rendering with Direct2D/Direct3D 10.


  • Support for the displaystyle attribute on the top-level <math> element has been added.
  • The interpretation of negative row numbers for the align attribute on <mtable> has been corrected.

Developer tools

  • The console object has a new dir() method, which displays an interactive list of the properties on a specified object.

Changes for Mozilla and add-on developers

See Updating add-ons for Firefox 8 for a guide to changes you're likely to have to make your add-ons compatible with Firefox 8.

Note: Firefox 8 requires that binary components be recompiled, as do all major releases of Firefox.



The new methods Components.utils.createObjectIn() and Components.utils.makeObjectPropsNormal() have been created to make it easier to create objects in specific compartments.

  • You can now instantiate DOM File objects from component code by doing new File, instead of having to instantiate an nsIDOMFile directly.
  • The nsTPtrArray array type has been removed. Its functionality is now all available on nsTArray, which now offers the SafeElementAt() method when instantiated using a pointer type.


It is no longer possible to access XPCOM objects from ChromeWorkers. XPConnect has been disabled in worker contexts as of Firefox bug 649537.


  • A bug in document.execCommand() that occurred when calling it on the value of contentDocument has been fixed. Since Firefox 3, this has resulted in errors instead of working correctly.
  • Bootstrapped add-ons can now load chrome using a chrome.manifest file.
  • XUL images now shrink down with the same ratio in both directions when specifying maximum sizes.

Changes to the build system

  • The following build configuration options have been removed:
    • --enable-timeline
    • --disable-storage
    • --necko-disk-cache
  • When compiling IDL files to headers, the header file jspubtd.h is automatically included when needed. Manual inclusions of jspubtd.h and/or jsapi.h in IDL files that use jsval or [implicit_jscontext] are no longer necessary.

Chrome registration

  • The platformversion flag can be used in the chrome.manifest to specify Gecko version compatibility.

Interface changes

  • The mozIJSSubScriptLoader.loadSubScript() method now loads scripts from the startup cache when possible.
  • The ownerWindow attribute has been removed from the nsIAccessNode interface.
  • The nsIDOMStorageWindow interface has been merged into the nsIDOMWindow interface.
  • All members of the nsIDOMWindowInternal interface have been moved into the nsIDOMWindow interface. The interface itself (with no members) remains available for compatibility until Firefox 9.
  • In order to improve performance, callback handling for asynchronous Places database updates has been changed. See the new mozIVisitInfoCallback.handleResult() and mozIVisitInfoCallback.handleError() methods, which replace the old single method for both error and success condition handling.
  • The KIND_MAPPED attribute of nsIMemoryReporter has been deprecated in favor of KIND_NONHEAP, new unit types UNITS_COUNT_CUMULATIVE and UNITS_PERCENTAGE have been added.
  • The nsIMemoryReporterManager interface has a new explicit attribute, which reports the total size of explicit memory allocations.
  • The nsIMemoryReporterManager interface has a new resident attribute, which reports the amount of physical memory used.
  • The nsINetworkLinkService interface has a new attribute, linkType. This attribute provides the type of network connection in use. All Operating Systems currently return LINK_TYPE_UNKNOWN. Android support was backed out due to perceived security concerns.
  • The nsISelection2 interface has been merged into the nsISelectionPrivate interface.
  • The nsISelection3 interface has been merged into the nsISelection interface.
  • The nsISessionStartup attribute state is now a jsval instead of a string, for performance reasons.
  • The nsIDocShell attribute isActive is now false for minimized windows.
  • The nsIDownloadHistory.addDownload() method now saves the target where the download is saved on the local file system.

Removed interfaces

The following interfaces were implementation details that are no longer needed:

  • nsITimelineService
  • nsIDOMHTMLIsIndexElement

The nsIWorkerFactory interface has been removed as well. Workers can still be created using the Worker and ChromeWorker constructors.

Other changes

  • When a window is minimized, un-minimized, or switches between full screen and windowed mode, it receives a sizemodechange event.
  • You can now use the extensions.autoDisableScopes preference to disable automatic installation of add-ons from specific add-on install locations.
  • The new mozSyntheticDocument property on Document objects lets you determine whether a document is synthetic (that is, something like a standalone image, video, or audio file) rather than a full, standard DOM document. This can be useful, for example, if you want to present different user interface in this situation (such as adding contextual items differently depending on this case).
  • You can now specify a filter when opening about:config; for example, "about:config?filter=sessionstore" will show only session storage related preferences.

See also