This is a video I found of a guy showing you how to make lumber out of firewood. I have not tried this but if you wanted some small pieces of lumber I can see that it would work. This might be good if you had some firewood made from a good quality hardwood and wanted it to make a small project.
This guy uses a jointer and a bandsaw but there are other cutting tools you could use to cut a piece of firewood into lumber. After you have some flat edges a table saw would probably work well to rip it into boards.
With any of this be very careful. Working with these irregular shaped pieces can be dangerous with power cutting tools so I suggest you not try this if you are not experienced and qualified. Actually I should take my liability disclaimer a step further and say don’t ever do this, ever, it’s too dangerous.