Dunno if you noticed dude, but there is a HUGE debate at the moment that the Berserker is overpowered. I perosnally feel he has been pushed too far the other way.
In terms of why 1 more tank shouldn't be added is for this 1 reason. Having too much ammo for the Firebug encourages very sloppy play, pretty much like the Zerker is now and the Sharpshooter used to be.
I tried the flamethrower in WTF mod (which gave it more ammo) and it was ludicrously boring cause you just mindlessly hosed down everything in sight with no fear of runing out of ammo. Firebugs who frequently use continous spray are not using the perk as it is intended, and they get punishing accordingly with ammo. Plus they also block their teammates view with constant strings of flame.
The class is all about using damage over time to your advantage and spraying down when needed. This is what makes the class interesting and fun.
So while I appreciate the train of thought, I would truly hate to see this change to the Firebug because it would ruin his position in the game, very much like the Berserker is doing now