Do a clean (re)install:

• Download a fresh Firefox copy from http://www.mozilla.com/firefox/all.html and save the file to the desktop.
• Uninstall your current Firefox version and remove the Firefox program folder before installing that copy of the Firefox installer.
• Don't remove personal data if you uninstall the current version.
• 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.

Your bookmarks and other profile data are stored elsewhere (not in the Firefox program folder) and won't be affected by a reinstall, but make sure that you do not select to remove personal data if you uninstall Firefox.

It would have been far too easy... When I try to uninstall, it asks me to restart the pc in order to complete a previous update, then everything is exactly like before! It's the update that failed, I can't understand how to cancel it!

Only have this problem on my Windows7 laptop since I ran Windows update on it yesterday. My XP netbook runs latest version fine. HAve uninstalled, deleted folder and reinstalled but still same XUL error

I run Avast antivirus and had the same problem. I found this fix on Avast's support site:

Open Avast Internet Security Open Real-Time Shields>Process Virtualization>Expert Settings Click 'delete contents'

Worked for me.