Do i have to update firmware to use cydia?

I have an Iphone 3G that i'm gonna be jailbreaking it is running IOS 4.2.1 with the firmware being 5.1.5.04 if i update it to 6.1.5.00 will i be able to restore it if anything goes wrong with the jailbreaking i'm not worried about the warranty since it expired and its basically being used as an ipod. Also do i even have to update it or can i just leave it alone and still use cydia

Added (1). I'm not going to be buying a newer Iphone for awhile as this one is working fine for me

Added (2). Yes that is what i mean do i have to update the baseband in order to even use cydia

5.15.04 and 6.15.00 are the baseband not firmware.
use whited00r to get some ios6 feature on older idevices.