HOW CAN I RESTORE ALL OF MY IPHONE SETTINGS FROM A BACKUP WITHOUT RESTORING THE FIRMWARE?

I jailbroke my iphone 3g, which was using firmware 3.1, in the routine of which I lost all of pictures, contacts, apps, etc. But these have been all saved in a backup file. However I think which if i revive the iphone routinely it will overwrite the firmware and it won’t be jailbroken anymore. Is there any approach which I can revive it from backup but restoring the firmware?
Does which have sense?
Thanks really most for any help.