actually, you're suppose to be immune to the first flash from your nades. after that it is the entire flash duration.
the fire nades eal good damage, in a small area within the flames. but you hear "hits" from them even when it doesn't eal amage. they're more like alarms than anything. give you an opportunity to detect approaching enemies, poison nae them (make them move SLOW) and run/uzi them to bitches