The More Fuels mod gives you the ability to use practically anything as a fuel source in a furnace. You can use items as simple as torches, cacti, or signs:

Or if you have bigger items you no longer need, you can burn these items for an incredibly long time. Things like beds, maps, fireworks and TNT will burn for several minutes giving you a heftier yield of smelted items per fuel source over standard coal.

And even more interesting with this mod is the ability to smelt rotten flesh into leather. Simply put it in the furnace with any fuel source and you’ll start to create leather. This is super handy if you happen to have an abundance of useless rotten flesh that you can ultimately make something useful out of now.

Check the wiki page here for a list of burnable resources and how long they burn for.