I'll help you out a little more: by setting a breakpoint to $73 when starting a game, you can find an instruction at $C3C7 ($3D7 in ROM) that loads $10 into the accumulator before writing it to $73.
So, just change $3D8 to whatever number you want and bingo, there's your starting fire value.
Furthermore, when you collect a fire icon, the game (at $CF1A, $F2A in ROM) checks your current fire count to see if it's reached $50 (the maximum), and adds $10 if not. So by messing with that routine, you could either add two powerups each time, or remove the check, allowing you to get crazy big flames.
By the way, I think bomb powerups are stored at $74 - I'll let you figure those out.