Bukkit/CraftBukkit — comment on commit 5b36c2f96ca0a7a4d151ece412a9ff33cff6fd36
File: src/main/java/net/minecraft/server/Entity.java line 325
By Cogito:

It doesn't really matter what the event is declared as, since it is created as an EntityCombustByBlockEvent it is such. If your instanceof check doesn't work then either the event is being created somewhere else, or your check is wrong.