Certain Firefox problems can be solved by performing a Clean reinstall. This means you remove Firefox program files and then reinstall Firefox. Please follow these steps:

Note: You might want to print these steps or view them in another browser.

1. Download the latest Desktop version of Firefox from http://www.mozilla.org and save the setup file to your computer.
2. After the download finishes, close all Firefox windows (click Exit from the Firefox or File menu).
3. Delete the Firefox installation folder, which is located in one of these locations, by default:
   • Windows:
     • C:\Program Files\Mozilla Firefox
     • C:\Program Files (x86)\Mozilla Firefox
   • Mac: Delete Firefox from the Applications folder.
   • Linux: If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it - see Install Firefox on Linux. If you downloaded and installed the binary package from the Firefox download page, simply remove the folder firefox in your home directory.
4. Now, go ahead and reinstall Firefox:
   1. Double-click the downloaded installation file and go through the steps of the installation wizard.
   2. Once the wizard is finished, choose to directly open Firefox after clicking the Finish button.

Please report back to see if this helped you!

Do a clean reinstall and delete the Firefox program folder before (re)installing a fresh copy of the current Firefox release.

Download a fresh Firefox copy and save the file to the desktop.

• Firefox 21.0.x: http://www.mozilla.org/en-US/firefox/all.html

Uninstall your current Firefox version, if possible, to cleanup the Windows registry and settings in security software.

• Do NOT remove personal data when you uninstall your current Firefox version, because all profile folders will be removed and you will lose all personal data like bookmarks and passwords from profiles of other Firefox versions.

Remove the Firefox program folder before installing that newly downloaded copy of the Firefox installer.

• (32 bit Windows) "C:\Program Files\Mozilla Firefox\"
• (64 bit Windows) "C:\Program Files (x86)\Mozilla Firefox\"

• It is important to delete the Firefox program folder to remove all the files and make sure that there are no problems with files that were leftover after uninstalling.
• http://kb.mozillazine.org/Uninstalling_Firefox

Your bookmarks and other personal data are stored in the Firefox profile folder and won't be affected by an uninstall and (re)install, but make sure that "remove personal data" is NOT selected when you uninstall Firefox.

• http://kb.mozillazine.org/Profile_folder_-_Firefox
• http://kb.mozillazine.org/Profile_backup

• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Clean_reinstall

Unfortunately, this did not seem to work. I still get the xpcom.dll error. Unless there is more specificity to performing a clean re-install, or I am missing some crucial step in the process, then I am afraid I won't be able to run Firefox.

I am having the same problem. I have uninstalled Firefox and deleted the C:\program files (x86)\mozilla directory before reinstalling.

OS is Windows 7 64 bit.

One more thing this only occurs when Outlook or Word is open, also even when I open Outlook in safe mode so no add ons are running.

I'm running Mozilla v.22 btw

I also have this problem with Windows 7 and firefox 23.0.1 I've done a clean install after deleting the firefox directory as recommended. It doesn't happen all the time. I can launch firefox with no error, shut it off, and relaunch immediately (without starting any other programs) and get the error.

Or it can go the other way, launch, have error, shut off, re-launch OK.

The error doesn't seem to be causing any loss of functionality of Firefox.

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe Mode start window.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

I updated the Quicktime plugin and the problem seems to have gone away. (Not 100% certain as it was intermittent). Quicktime had been flagged as out of date. This was the only plugin that was out of date.

Update: This wasn't it as the problem is back. Firefox reports it is trying to start nsAppShell and failing.