You need to do a clean reinstall.

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

• Firefox 5.0.x: http://www.mozilla.com/en-US/firefox/all.html

• Uninstall your current Firefox version.
• Do not remove personal data when you uninstall the current version.

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

• 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 in the Firefox Profile 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.

Firefox 5 on Vista, did clean install. Worked once then same error message. Will not work in safe mode. Did reinstall over current install, same thing. No plugins installed. Firefox 4 was running fine till forced update to Firefox 5. What is this Microsoft QA, slap on some new code and send it out to be tested on everyone for bugs? Solution do a clean removal and load a different browser.

Dug a little further. FF5 does not work with Zone Alarm or Protector Suite QL which comes with Toshiba Satellite laptops. Deleted them both and loaded AVG Free. FF5 seems to work with no problems. Protector Suite 2011 claims to work with FF5 but you have to buy it. There is no upgrade offered through Toshiba.