DO NOT USE THE C BUTTON!!! This is Fire 2 on the Amiga and is used by many games! If you are going to assign a button to be used as the UP direction for your games, please use A!
Maybe it was A. Still have the joystick (it's a Quickshot for the Genesis) but haven't used it in a long time.
But, yeah, A was "jump", B and C were Fire1 and Fire2 or how ever it worked out. Yeah, modding "UP" to a Fire button would be bad!! :-) Well it might be interesting anyway.
I always remember having the 2 normal fire buttons though on my joystick/pads. People that were using 1 button controllers must had those left over from their C64's ;-)