Depends what you are looking for. There is no "one button wonder" macro.
If you only want to use one button you can do something like this:
#showtooltip
/cast [mod:shift] Pyroblast; [mod:ctrl] Living Bomb; Fireball
This should spam fireball if you keep spamming it, Pyroblast if the modifier shift is down, and Living Bomb if the modifier ctrl is down.
But imo this is overkill, i would rather use the following binds as i use atm while playing fire:
1: Fireball
2: Scorch
3: Pyroblast
4: Fire Blast
q: Flamestrike
ctrl-q: Blast Wave
e: Living Bomb
shift-e: Combustion
I hope it helped somewhat .
- Nitrak