This issue can happen if the updater wasn't able to update all the files.

Do a clean (re-)install:

• Download a fresh Firefox copy and save the file to the desktop.
• Firefox 3.6.x: http://www.mozilla.com/en-US/firefox/all-older.html
• Firefox 4.0: http://www.mozilla.com/firefox/all.html

• 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.

I found a similar message on this board yesterday and did just what you suggested and it worked. Thank you for the info.