I am a carpenter, I have used a miter-saw to cut things almost twice as thick as the depth of the saw blade just by flipping the wood around to cut the next half. Most miter saws max depth is about 3.25" but if you cut one side (3.25") then the other side (3.25") you can cut 6" or so.
Must be it getting cold where you are at, 30 degrees F here right now just put some wood in the fire myself. I'd cut firewood with one, just watch your fingers and if the saw binds or gets hot ease off and give it a break.
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.
For some jobs, it is the right tool. Like cutting 20" long split wood down to 10" because your new stove only takes 12" Or cutting wrist sized limbs to length.
But they Can be dangerous, if the wood doesn't fit against the fence it can move and bind.
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