If reinstalling doesnt't work, it could be because the new installation of firefox still uses the old installation's application data folders and files that are stored for your user account.
After trying what AntRid suggested you could try the following if it didn't help:
Navigate to the folder "(drive letter)\Documents and Settings\username\Application Data\Mozilla\Firefox" and delete the profiles folder.
Then go to (drive letter)\Documents and Settings\username\Local Settings\Application Data\Mozilla\Firefox and delete the profile folder there too.
Keep in mind that this will probably also make you lose your browser history, bookmarks, cookies, saved passwords, and custom browser layouts, so be prepared!
Some of the aforementioned folders may be marked as hidden or system folders, and could therefore be invisible to you if you try to navigate there manually. If you type in the folder names in the address bar, you should still be able to get into them.
