I don't believe you will ever make a "good" profit out of selling firewood. But what you need to think of is
1. Do you have a reliable renewable source of wood that you have complete access to.
2. Is it close enough to your demand market to keep your distribution costs low.
3. Can you work 16 hours a day, 7 days a week for months at a time?
4. Are you mechanically adept in order to keep your down time due to mechanical failures at a minimum. (they will happen, take my word for it)
5. Are you honest? You will soon go out of business if you are not.
6. Can you afford the equipment needed to produce lots of firewood fast. Check out the internet for fast and efficient splitting machines.
7. What is the competition in your area. No sense starting up with 5 other sellers around you.
Selling firewood is a very labor intensive business. There are so many hidden costs (fuel, insurance, etc) that have to be taken into consideration that I would not start until you have done a lot of research.
One final note, with Craigslist giving away free wood, and many weekend beerdrinking money sellers (who offer their wood at 1/2 your cost), you best love what you are doing because you will get very frustrated if you do not.
You will not get rich selling firewood.