I'm most positive that stnding in a groundfire will deal the groundfire damage as well as the afterburn damage at the same time.
take kodachi for example. When kodachi shoots, it deals 3 sorts of damage.
-Impact damage(70 damage explosion) like a very weak higher AOE riot cannon
-AOE DOT damage(36 DPS) the ground fire
-apply DPS burn(15 DPS) the 2 second fire linger after leaving ground fire
I've heard from someone that the groundfires deal less damage towards the end of their durations, I can't confirm it's hard to feel that damage loss. a unit can only have 1 15 DPS fire on it at a time, but groundfires will stack.so a drive by of like 5 kodachis to a factory will burn the construction zone for like 200 damage a second(36*5+15), rendering it inoperable and frighteningly losing HP.