I guess if you removed all enemy AI and changed their sprites to different sized fires, it could be feasible. But then, you would need to have the fires spread and destroy objects and hostages.

Unfortunately, it doesn't sound very feasible.

it seems like it would need a complete overhaul of some systems

if you want something with firefighters, get everyday heroes

Nice idea, although hard to implement as Kakroom said. Still I can recommend the co-op boardgame Flash Point Fire Rescue. I am even considering using some of their game boards / houses to make some levels for Doorkickers. But it will be a while until I get to that, currently working on a themed non-lethal mod based on the spy comedy TV show Chuck.