Recently ran the climactic encounter with a Boss Villain. The PCs were storming his hide-out, which was guarded by a small army of grunts and the Boss Villian's badass bodyguard, a chainsword-wielding mass of surgically-altered muscle which, due to a botched operation, could feel no pain (represented by an unnaturally high Toughness) and thus was utterly fearless. As the grunts and the PCs traded shots, the Bodyguard moved into possition to charge. Before the Bodyguard could charge, a character hit him with his favorite weapon, a firebomb, setting the big baddie ablaze. The rules say that when someone is on fire they can take no action other than try to put the fire out. After failing a couple Agility tests to do so, the resultant Fatigue levels made it impossible for him to extinguish the flames; thus the scaryest opponent spent the entire encounter spinning in place and very slowly burning to death, as the players picked off the grunts at their leisure.
So, my question is this: are there any rules for the fire going out on its own? I couldn't find any in the rules for Firebombs or for burning. Do I need to make up house rules for this? I'm dreading every major encounter fizzling out due to some bad Agility tests on the villains part... I considered stating that the Bodyguard would be immune to the "do nothing else" effect of being on fire due to his nerve damage, but I felt that that would sound too arbitrary to the players. Can even Daemons be "shut down" with this cheap, common weapon?