if everything else failed, you should remove your prefs directory in
Code:
C:\Documents and Settings\<login name>\Application Data\Mozilla
Be warned that it will remove really all your prefs, including bookmarks. You might want to save the file
Code:
C:\Documents and Settings\<login name>\Application Data\Mozilla\Firefox\Profiles\<random string>\bookmarks.html
elswhere before deleting the whole folder
restart firefox, it should be clean as new