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Release Notes Mozilla Developer Preview (1.9.3 alpha)

The Mozilla Developer Preview of Gecko 1.9.3 (alpha) is an early developer milestone containing new features of the Gecko layout engine. It is being made available for testing purposes only, and is intended for web application developers and our testing community. Current users of Mozilla Firefox should not use this Mozilla Developer Preview.

New Features and Changes

Gecko 1.9.3 alpha introduces new features which can be tested by using this Mozilla Developer Preview. Many of these features are still in development, and while they will likely appear in some future version of Mozilla Firefox, some may be in earlier releases than others.

Alpha 5: 14-June-2010
Addons Manager New Addons Manager (placeholder UI only, will be updated before final release). See Extension Manager Redesign for more information.
Tabs on Top You can change the location of your tabs by selecting "Tabs on Top" in the View menu to try it out and see if you prefer your tabs on top of your toolbar.
Video Support for WebM!
Hardware Acceleration Firefox now uses hardware acceleration for video playback.
Plug-Ins Cocoa NPAPI support (used by Flash 10.1 and Apple's newest Java plugin on Mac).
HTML New HTML parser (HTML5). Support for more HTML5 form controls. Support for FormData.
CSS Support for :-moz-any().
Platform ChromeWorker with jsctypes support
Lazy frame construction for faster dynamic pages.
JS performance improvments.
Memory usage Loading the URL about:memory now shows how much memory is used by different parts of Firefox.
Build 64-bit Mac and Linux versions now available.
Alpha 4: 12-April-2010
Privacy CSS :visited selectors have been changed to block ways that websites can quickly check a user's browsing history. See for more information.
User Interface Currently loaded web pages are shown in the location bar autocomplete list, allowing switching to existing tabs. See bug 480350 for details.
Stability The beta version of Adobe Flash is now run in a separate process on Mac OS X Snow Leopard (10.6). Visit the Adobe website to install the latest beta version of Flash.
SVG SVG attributes which are mapped to CSS properties can now be animated using SMIL. See bug 534028 for details.
Video Full-screen video on Windows is now rendered using hardware acceleration when available. See Bas Schouten's blog for details.
Plug-Ins Mozilla has implemented the Core Animation rendering model for plugins on Mac OS X. Plugins which also support this rendering model can now draw faster and more efficiently.
HTML HTML <textarea> form controls can be resized by the user. See bug 442228 for details.
Performance Linux builds are now built with -fomit-frame-pointer, improving page load times on average by 4%. For more information, see bug 554364.
Alpha 3: 17-March-2010
Performance An experimental Direct2D rendering backend on Windows is available, turned off by default. For information and instructions on enabling this experimental feature, see Bas Shouten's post.
JavaScript Significant API improvements are available for JS-ctypes, a foreign function interface for extensions. ctypes.jsm now supports complex types including structures, pointers, and arrays. See Dan Witte's post for more information.
Stability Mozilla now uses an infallible allocator: if a request for memory cannot be fulfilled, the allocator aborts. This allows for simplification of the Mozilla code, as well as preventing a class of security errors with unchecked null pointers. See bug 441324 for details.
Plugins Additional fixes for multi-process plugins:
  • crash and hang fixes;
  • the Foxit Reader and Octoshape plugins work correctly;
  • Performance improvements in the delivery of network streams.
Alpha 2: 3-March-2010
Plugins On Windows and Linux, plugins (such as Flash and Silverlight) are now isolated from Firefox. Plugin crashes will not kill Firefox itself, and unresponsive plugins are automatically restarted.
Security The SSL security system has been changed to fix a renegotiation flaw. For technical details, see the newsgroup posting announcing the change.
Performance Link history lookup is now performed asynchronously on a thread. This results in less I/O during page load and improves overall browser responsiveness.
Performance Loading the HTML5 specification no longer causes very long browser pauses. See bug 526394 for details.
JavaScript The JavaScript engine has many improvements: string handling is improved, faster closures, and some support for fast tracing and JIT of recursive functions.
Performance Strings are not copied between the main DOM code and web workers, improving performance for threaded JavaScript which moves large pieces of data between threads.
HTML The placeholder attribute for <input> and <textarea> is now supported.
User Interface The stop and reload buttons have been merged when they are adjacent on the toolbar. See bug 343396 for details.
SVG Support for SMIL Animation in SVG. Support for animating some SVG attributes is still under development and the animateMotion element isn't supported yet.
Stability Crash reports from plugin processes are now submitted automatically. Crash report submission can be disabled in Firefox preferences (Advanced / General / Submit crash reports).
Performance Repainting HTML in SVG <foreignObject> is faster. See bug 541188 for details.
Alpha 1: 10-February-2010
CSS Support for CSS Transitions. This support is not quite complete: support for animation of transforms and gradients has not yet been implemented.
WebGL Support for WebGL, which is disabled by default but can be enabled by changing a preference. See this blog post and this blog post for more details.
DOM Support for the getClientRects and getBoundingClientRect methods on Range objects. See bug 396392 for details.
DOM Support for the setCapture and releaseCapture methods on DOM elements. See bug 503943 for details.
HTML Support for the HTML5 History.pushState() and History.replaceState() methods and the popstate event. See bug 500328 for details.
CSS Support for the -moz-image-rect() value for background-image. See bug 113577 for more details.

We are interested in feedback on any bugs or missing capabilities in these new features. Some of the changes will affect web and platform compatibility. For detailed information about compatibility changes in Gecko 1.9.3, please read Upcoming Firefox features for developers.

Downloading and Installing

System Requirements

Before installing, make sure your computer meets the system requirements.

Please note that Mac OS X 10.5 or higher is required for this preview.

Downloading the Mozilla Developer Preview provides this Mozilla Developer Preview for Windows, Linux, and Mac OS X in English.

For builds for other systems and languages not provided by, see the Contributed Builds section at the end of this document.

Installing the Mozilla Developer Preview

Once you have downloaded the Mozilla Developer Preview, follow these instructions to install:


Double click the MozillaDeveloperPreview Setup 3.7 Alpha 5.exe installer to start the install.

Mac OS X

Double click the MozillaDeveloperPreview 3.7 Alpha 5.dmg (32-bit) or MozillaDeveloperPreview 3.7 Alpha 5.dmg (64-bit) Disk Image to uncompress and mount it. Your browser may have already done this for you. Double click the MozillaDeveloperPreview Disk Image to open it in Finder and drag the MozillaDeveloperPreview application onto your hard disk. Do not double click the icon in the disk image! Be sure to drag the MozillaDeveloperPreview application out of the disk image and onto your Hard Disk before running it. Drag the icon to your Dock if you want it to appear there.


Extract the mozilladeveloperpreview-3.7a5.tar.bz2 32-bit) or mozilladeveloperpreview-3.7a5.tar.bz2 (64-bit) tarball and run ./firefox:

tar -jxvf mozilladeveloperpreview-3.7a5.tar.bz2
cd firefox

Other Platforms

Extract the compressed archive and run firefox

Uninstalling the Mozilla Developer Preview

To uninstall the Mozilla Developer Preview, follow these instructions:


From the Start menu, choose Control Panel. When the Control Panel appears, double click Add/Remove Programs. Find "Mozilla Developer Preview (3.7a5)" in the list and click Remove to uninstall.

Mac OS X

Drag the MozillaDeveloperPreview application to the Trash.


Remove the firefox folder.

These instructions leave your profile in place in case you install Firefox again in the future. If you wish to remove your profile folder, remove the location described below in the "Profile Folder" section. Note that by doing this you are destroying all of your downloaded Extensions and Themes, Bookmarks, saved passwords, settings and other information and should you decide to use Firefox again you will be starting with a clean profile.

Other Information

Profile Folder

The Mozilla Developer Preview will use the same store of bookmarks, extensions, history, and other data as a release version of Firefox. See the Mozilla support article on profiles for more information.


Add-ons installed under previous versions of Firefox may be incompatible and/or require updates to work with this Mozilla Developer Preview. Please report any issues to the maintainer of the extension. When you install the Mozilla Developer Preview, all of your Add-ons will be disabled until the Mozilla Developer Preview determines that either a) they are compatible with the Mozilla Developer Preview release or b) there are newer versions available that are compatible.

Known Issues

This list covers some of the known problems with the Mozilla Developer Preview of Gecko 1.9.3 Alpha 5. Please read this before reporting any new bugs, and watch for updates as new bugs are discovered.

All Systems

  • Add-ons installed under previous versions of Firefox may be incompatible with the Mozilla Developer Preview of Gecko 1.9.3 Alpha 5.


  • The edge of the General pane in Preferences is cut off due to the long name "Mozilla Developer Preview". (bug 545027)
  • Windows Aearo Glass and fullscreen mode in Firefox doesn't work well together. More information in bug 567742.
  • Viewing an HTML5 video (<video>) in full screen mode can now play back the video using hardware acceleration. This hardware accelerated playback is fairly early in its development cycle and is known to not work with certain graphics hardware. If you run into problems, setting the preference "mozilla.widget.accelerated-layers" to false will disable hardware acceleration completely. To make Firefox use OpenGL instead of DirectX, set the preference "mozilla.layers.prefer-opengl". If you do run into problems, please let us know in Bugzilla.

Linux and Unix systems

  • Plugins which use the older Xt drawing model will not work (bug 544088). A workaround for two common plugins (VLC and Acrobat Reader) has been implemented so that these plugins will run in Firefox instead of a separate process.
  • Firebug is known to cause 64-bit versions of Firefox to crash (bug 570990) on Linux.

Mac (32-bit and 64-bit) builds

  • Viewing an HTML5 video (<video>) in full screen mode can now play back the video using hardware acceleration. This hardware accelerated playback is fairly early in its development cycle and is known to not work with certain graphics hardware. If you run into problems, setting the preference "mozilla.widget.accelerated-layers" to false will disable the hardware acceleration. If you do run into problems, please let us know in Bugzilla.

Mac 64-bit builds

  • Debug symbols are not available for 64-bit builds yet, which means crash reports from 64-bit Mac builds reported to crash-stats will be significantly less useful to Mozilla developers.


  • Poorly designed or incompatible Add-ons can cause problems with your browser, including make it crash, slow down page display, etc. If you encounter strange problems relating to parts of the browser no longer working, the browser not starting, windows with strange or distorted appearance, degraded performance, etc, you may be suffering from trouble with your Add-ons. Restart the browser in Safe Mode. On Windows, start using the "Safe Mode" shortcut created in your Start menu or by running firefox.exe -safe-mode. On Linux, start with ./firefox -safe-mode and on Mac OS X, run:

    cd /Applications/
    ./firefox-bin -safe-mode

    When running in Safe Mode, you can disable the add-ons that are causing trouble and then restart to try again.

If you uninstall an extension that is installed with your user profile (i.e. you installed it from a web page) and then wish to install it for all user profiles using the -install-global-extension command line flag, you must restart the browser once to cleanse the profile extensions datasource of traces of that extension before installing with the switch. If you do not do this you may end up with a jammed entry in the Extensions list and will be unable to install the extension globally.

If you encounter strange problems relating to bookmarks, downloads, window placement, toolbars, history, or other settings, it is recommended that you try creating a new profile and attempting to reproduce the problem before filing bugs. Create a new profile by running Firefox with the -P command line argument, choose the "Manage Profiles" button and then choose "Create Profile...". Migrate your settings files (Bookmarks, Saved Passwords, etc) over one by one, checking each time to see if the problems resurface. If you do find a particular profile data file is causing a problem, file a bug and attach the file.

Frequently Asked Questions

  1. What can I do to help?

    We need help from developers and the testing community to provide as much feedback as possible to make the Mozilla Developer Preview even better. Please read these notes and the bug filing instructions before reporting any bugs to Bugzilla. You can also give us your feedback through this feedback form.

    If you see the crash reporter dialog, please try to provide a detailed comment indicating what you were doing at the time of the crash.

    If you're interested in helping us test pre-release builds, you might want to join our beta test mailing list.

  2. Why haven't you responded to the mail I sent you?

    Use the newsgroup. The Firefox team reads it regularly, and your email may have gotten lost.

  3. Where can I get extensions and themes (add-ons)?

    Extensions and Themes can be downloaded from Firefox Add-ons.

  4. Who makes the Mozilla Developer Preview?

    Lots of people. See Help->About Mozilla Developer Preview, Credits for a list of some of the people who have contributed.

  5. Where's the Mozilla Developer Preview source code?

    A tarball of the Mozilla Developer Preview source code is available for download. The latest development code can be obtained by Mercurial. Please follow the build instructions.

  6. Where is the mail client?

    The Mozilla Developer Preview works with whatever mail client is the default on your system. However, we recommend Mozilla Thunderbird, our next-generation email client and the perfect complement to Firefox.

Contributed Builds

These are unofficial builds and may be configured differently than the official builds. They may also be optimized and/or tested for specific platforms.

Localized Builds

Many localized builds are now produced and distributed by on behalf of their authors.