I just jailbroke/unlocked an old iPhone on a computer that it does not normally sync to.

When I plugged it back into the computer it normally sync to it asks whether to setup as a new phone or restore from backup.

Does restore from backup also restore the old firmware?

Can I just click setup as new phone and sync to get back all my contacts/calendar etc.?

Thanks

restore from backup doesn't restore the old firmware only the things that it backed up, like sms, contacts, camera roll, etc...If you setup up as a new iphone you can sync your things back up, but you won't get your text messages, and camera roll back.

your only option there is to hope that itunes kept the old backups so you can reset all settings and try again