I'm looking for a solution to the same problem for my woodchips. When I first made my woodchips block for 1.6 and earlier, I just edited BlockFire.java, but now I'm trying to use Forge, and I'm finding everything a lot harder to do. I found several methods in Block.java: getFlammability, isFlammable, getFireSpreadSpeed; but for some reason I can't override them in my modblock class. I guess it might have something to do with protected variables. The Forge documentation says that they return Flammability and FireSpreadSpeed values, but I think this might be wrong, and they're simply returning the blockID of the block in question, which is then picked up by BlockFire.java in its setBurnRate method, at which point the blockIDs are matched with Flammability and SpreadSpeed values. I thought I might cheat by popping in planks, and tricking fire into thinking that my woodchips were planks, but that doesn't change the fact that blockFireSpreadSpeed and blockFlammability are protected.