Try to delete the file profiles.ini to force Firefox to create a new default profile.

See also http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

i tried deleting the file but my firefox still cant work.

i tried to create a new profile but the firefox wont open at all. instead the mozilla crash reporter popped up.

You can try to rename (move) the Mozilla or Mozilla\Firefox folder if you have other Mozilla Applications (Thunderbird).

• Vista/Windows 7: C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\Profiles\<profile>\
• Windows: %AppData%\Mozilla\Firefox\Profiles\<profile>\

"Application Data" in XP/Win2K and "AppData" in Vista/Windows 7 are hidden folders, use %APPDATA% in the File name field.
See http://kb.mozillazine.org/Show_hidden_files_and_folders