Has the drive ever worked normally with your Mac? If so, can you remember what happened that may have affected the drive? Are you using an older enclosure? Both of my Firewire/USB 2.0 combo enclosures (from different companies) go to sleep and wake up with my G4 iBook.
I do not see how reformatting would address your problem. You can use Disk Utility or another utility to check for problems with the drive, but I wouldn't reformat unless you have some evidence that it will fix your problem, or you don't mind possibly wasting some time if it doesn't do any good.
Not all Firewire drives are bootable; for example, previous-generation Iogear drives are not bootable. If the drive/enclosure specs do not specifically state that it is bootable, it is best to inquire first before making a purchase.