Do a clean (re-)install:

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

• Firefox 4.0.x: http://www.mozilla.com/en-US/firefox/all.html
• Firefox 3.6.x: http://www.mozilla.com/en-US/firefox/all-older.html

• Uninstall your current Firefox version and remove the Firefox program folder before installing that copy of the Firefox installer.
• Do not 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.

Went to uninstall in control centre, will not allow an uninstallbecause it demands a reboot of the machine to complete the previous installation.

If you can't uninstall via the Control Panel then you can remove or rename the Firefox program folder manually.

See:

• http://kb.mozillazine.org/Uninstalling_Firefox