Brikwars 2010 has fire rules. Basically, fire comes in d4s as damage and whenever you make a fire attack, any d4 that comes up higher than the target's armor level (which is 1/2 for minifigs) ignites the target and adds a d4 of ongoing fire damage. This d4 is also taken as a penalty to armor and skill whenever they are used, and though fire doesn't cause anyone to move slower, they actually MUST use all of their movement points to run around screaming if they have any leftover.
You could probably get away with using a d6 if you don't have any d4s. I mean sure it's more damage, and if you wanna do d6-2 or something that's up to you, but