Start in safe mode with command prompt.
go to the firefox folder.
delete all files with the del command i.e. for a file called xxx.xxx you'd type del xxx.xxx
If any other folders are there, once empty you can remove them with the rd command. i.e. for a folder called abc, type rd abc from outside that folder.
You must remove everything from the folder first.
type del Mozilla Firefox
This should rid you of firefox completely.