I'm for mixing 'zerker and firebug. In my mind, they're two perks that only mad men would imitate in real life. Thus, mixing them is fitting :3
Well, that and they both lack diversity. If firebug had another weapon, one that you can wield alongside to a chainsaw, you'd have a choice between full 'zerker, katana+flamethrower, chainsaw+(insert new FB weapon here), or full FB. Thus giving them/it diversity.
But one problem comes to my mind: would there be a nerf to their current habilities? Wouldn't it be overpowered to have fire nades, bonus to fire, un-grabbiness, increased resistance to damage, immunity to fire, move faster, bonus zed-time, faster melee, faster reload with FT, spawn with chainsaw and flamethrower (above weight limit, btw)...
In fact, the right question would be: how would it be balanced?