Sounds like something went pear-shaped somewhere along the line. I suggest reinstalling like this:

• Go to Download Firefox 4.0 and download a fresh installation file to the desktop.

• Then go to Add/Remove Programs, scroll down to Mozilla Firefox and remove it, choosing to keep your bookmarks, customizations etc., (don't checkmark the box).

• Then reboot and delete the folder called Mozilla Firefox at this location: C:\Program Files\Mozilla Firefox

• Finally run the installation file you downloaded.

After you complete the installation, go to the Plugins Check page and update those if necessary.

Did you check your security software (firewall)?

A possible cause is security software (firewall) that blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program.

Remove all rules for Firefox from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox and the plugin-container process.

See:

• Server Not Found - Troubleshoot connection problems
• Configure firewalls so that Firefox can access the Internet

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

If you have a problem with the Firefox icon then you may have to reset the Windows icon cache.

• /questions/808930