Board Thread:Off Topic/@comment-38854975-20190905144044/@comment-36926079-20191011060353

In the first few episodes, grimm are shown to be wary of fire. While it is likely some grimm have a weakness or resistance to fire, we can assume that grimm react to fire in the same way as most animals.

That may be the problem- a person on fire would probably scream and roll around, while a grimm could and would likely simply charge toward you regardless of pain, setting everything underfoot on fire.