OK, yes that does make sense. having re-read a few passages grapple may not be appropriate. It could be used for smaller fires that wouldn't necessarily be immediately life threatening (arguably being on fire is a block against casual dinner conversation
. The way I see it working is this:
If the building is on fire then the proximity of the fire to the PC dictates whether the fire just prevents them doing certain things (like following an NPC through a collapsing doorway - a compel), or aids one party or another in their action (throwing a bolt of fire at the enemy by tapping into what's already there - invoke). If the fire is close enough to actually have an effect on a character then the Environmental damage rules kick in - which work almost identically to venom - bearing in mind that if the fire is severe enough then everyone in the scene suffers the same effects.
Unlike Venom the character has the chance to put themselves out by rolling on the floor etc, rather than waiting to just be treated by a doctor, with the strength of the fire being the attack roll. The attacks also may not just stop when a character is taken out if there are still consequence slots available.
Since the environmental damage rules say to treat the environment like an actual opponant you may also allow the fire itself to maneuver to add the aspect of ON FIRE to the character and then tag it to add more damage, but it's more likely to be used by someone attacking you saying that you're so distracted by the fire they get a bonus to their hit. Or you could invoke it to aid in a grappling maneuver - nobody likes to be that close to flamey boy! Or compelled when you're about to roll on the floor to put out a flaming sleeve and you notice that pool of gasoline creeping across the floor towards you...
It's not perfect, but the system isn't supposed to be a crunchy true life simulation.
Edited to make Morfedel happy!