Using "Forget About This Site" will remove everything from that domain like bookmarks, cookies, passwords, cache, history, and exceptions, so be cautious if you need to keep passwords or other data from that site and you can't recover from that unless you have a backup of the affected files.

It doesn't have any lasting effect, so if you revisit such a 'forgotten' website then data from that website will be saved once again.

this is a problem and Weakness in firefox the programers of firefox should fix this probelm