Here's an ancient screenie collection of a firefighter mod i did for our Maam-Sim B-25 in FS9. Easy peasy however the problem was that a simulated fire could be started anywhere but just no chance to kill it no matter bombarding it with retardant or water all day.
Later a payware fire proggy came along ( FSX ?..) that made it possible. Good fun ! I bet there's just no reason we can't have that in MSFS as well as fertilizer instead of retardant or water. It just takes so long !
Nontheless, i'd say lets worry about instant replay, fly-by camera and decent water spray/wake effect first. ( i do see some more or less decent smoke effects turning up lately, that's a good start ! ;-)