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all you have to do is

1)Create a folder on your usb called PS3 (all in capitals)
2)In that folder create another folder called UPDATE (all in capitals)
3)then put the original firmware in there (Either get it of playstation.com or This link: You must login or register to view this content.
4)then you need to put your ps3 in safety mode by following these instructions.

1)turn ps3 so you have the red light (so dont turn off at the back.
2)hold the red button until the ps3 turn on then off again.
3)hold the red button down again until you get a 2 beeps
4)now let your ps3 boot
5)go to system update at the bottom
6)put your usb in and click it
7)now wait for it to prepare (takes a few minutes)
then it will install the update and you are done (plus ftp will still work obviously because it is signed) woooo!!!

Hoped this helped

Most people know how to do this but it will help others whos new to the Playstation 3 hack community