As people's responses have indicated, there are a lot of variables. But the short
answer is, yes.
Miter saws vary a lot. My Nobex (a Christmas present of sorts to myself this year) can cut over 6" with no problem. The blades vary, too: I have a 16 tpi (teeth per inch) blade that makes fairly rough cuts but goes quick, but many saws in miter boxes have a higher tpi for smoother but slower cuts. You could definitely cut firewood with the right setup.
That said, I can't help but think that if I were
cutting firewood with a saw, it might be easier to do it without the box. That's because you could choose your angle and position for speed, which would be most important. If all I want to do is get through something, I prefer to have it a couple feet off the ground with my foot on it. A miter box controls things and goes a bit slower, and if you don't need the precise angles it makes, well, speed is better. Just a thought.