MineTweaker also allows modifying furnace recipes and furnace fuels using a few simple commands.
Do you believe smelting ores in a furnace is too easy? You can fix it as such:
furnace.remove(<minecraft:gold_ingot>); // alternatively, you can specify the input item furnace.remove(<*>, <minecraft:iron_ore>);
Found it sad you can't turn charcoal into real coal? Easy fix:
furnace.addRecipe(<minecraft:coal:0>, <minecraft:coal:1>); // or, if you want to generate 0.5 XP by doing so furnace.addRecipe(<minecraft:coal:0>, <minecraft:coal:1>, 0.5);
Want to make items burnable? Set their fuel value wit the setFuel function:
The same method can be used to set existing fuel - EXCEPT for vanilla fuels since those are, sadly, hard-coded. Additionally (with the exception of vanilla fuels) you can remove fuels by setting their fuel value to 0.