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The Mozilla Developer Preview of Gecko 1.9.3 alpha 1 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.

These Release Notes cover what's new, download and installation instructions, known issues and frequently asked questions for this Mozilla Developer Preview release. Please read these notes and the bug filing instructions before reporting any bugs to Bugzilla.

Give us your feedback through this feedback form.

Changes in this Development Milestone

Gecko 1.9.3 Alpha 1 introduces several 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 versions than others.

We are interested in feedback on any bugs or missing capabilities in these new features.

It also contains several other significant changes:

  • On Mac OS X, we render text using Core Text rather than ATSUI.
  • We rewrote major parts of the code for handling scrolling. See bug 526394 for details.
  • We rewrote the way a snapshot of a document is taken in order to print or print preview. See bug 487667 for details.
  • We made significant changes to table border handling. See bug 452319 and bug 43178 for details.
  • We made various architectural changes to improve Web page performance.

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. For a full list of changes, see this list (it's big).

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

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

For builds for other systems and languages not provided by mozilla.org, 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 1.exe installer to start the install.

Mac OS X

Double click the MozillaDeveloperPreview 3.7 Alpha 1.dmg 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-3.7a1.tar.bz2 tarball and run ./firefox:

tar -jxvf mozilladeveloperpreview-3.7a1.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 (3.7a1)" 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 stores your user data in one of the following locations:

Windows Vista Users\<UserName>\AppData\Roaming\Mozilla\Firefox
Windows 2000, XP Documents and Settings\<UserName>\Application Data\Mozilla\Firefox
Mac OS X ~/Library/Application Support/Firefox
Linux and Unix systems ~/.mozilla/firefox


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 1. 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 1.


  • The edge of the General pane in Preferences is cut off due to the long name "Mozilla Developer Preview". (bug 545027)

Mac OS X

  • The Java plugin does not work.
  • Part of the About dialog is cut off. (bug 544390)

Linux and Unix systems


  • 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/MozillaDeveloperPreview.app/Contents/MacOS/
    ./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'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 Mozilla.org builds. They may also be optimized and/or tested for specific platforms.

Other Systems

Builds for platforms other than the trio officially offered by Mozilla.org (if any) can be found on the FTP site.

Localized Builds

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

Builds that have not yet been certified as official localizations of the Mozilla Developer Preview of Gecko 1.9.3 Alpha 1 (if any) are available by browsing the FTP site.