If you recently installed a new version of FF with the old one still on your computer, this would cause you a problem. You must uninstall both versions then reinstall the on you wish for.
The seccond thing i would say to check is the icon on your desktop. Right click and check it's "target" and "start in folder" They should be pointing to (target)the firefox.exe (ie "C:\Program Files\Mozilla Firefox\firefox.exe"), and (start in folder) whould be pointing to the folder in which the exe is located (ie "C:\Program Files\Mozilla Firefox")
Other than that, i would just un-install and reinstall. Cause im not sure what's going on unless your computer has caught a bug.