Completely Uninstall Firefox

Start >> Control Panel >> Add/Remove Programs >> Uninstall Firefox. During the uninstallation, check the Remove my Firefox personal data and customizations box. (Remember to backup any data first)

Navigate to C:\Program Files\Mozilla Firefox and delete anything in that folder if there is any.

Delete the following directories if they still exist:
C:\Documents and Settings\%User Profile%\Application Data\Mozilla
C:\Documents and Settings\%User Profile%\Local Settings\Application Data\Mozilla


Next, delete the registry keys. Start >> Run >> Regedit, Delete the following keys:

HKEY_CLASSES_ROOT\FirefoxHTML
HKEY_CLASSES_ROOT\FirefoxURL\
HKEY_CURRENT_USER\Software\Classes\Applications\firefox.exe
HKEY_CURRENT_USER\Software\Mozilla
HKEY_CURRENT_USER\Software\MozillaPlugins
HKEY_LOCAL_MACHINE\SOFTWARE\Mozilla
HKEY_LOCAL_MACHINE\SOFTWARE\mozilla.org
HKEY_LOCAL_MACHINE\SOFTWARE\MozillaPlugins