Uninstall Mozilla Firefox Completely (Windows OS) (How to)
**Before proceeding backup your bookmarks.
Go to the control panel, click add/remove or programs and features, locate Mozilla Firefox and remove/uninstall.
Delete installation directories C:\Program files\Mozilla Firefox or, on 64-bit Windows, C:\Program Files (x86)\Mozilla Firefox
Enable View Hidden Files and Folders.
Locate folders named "Mozilla" under Documents and Settings or AppData
Open the Registry Editor by pressing the Windows Key + R type regedit and click OK.
HKEY_CURRENT_USER\Software\Mozilla and delete it by right clicking it and selecting delete.
HKEY_LOCAL_MACHINE\SOFTWARE\Mozilla and delete it by right clicking it and selecting delete.
You'll also find an entry titled "MozillaPlugins" you don't need to delete it but if you do it won't harm anything.
Click File, Exit
Restart your PC
Install Mozilla Firefox