As with GTA games prior to San Andreas, you can participate with side missions that involve vehicles. The Firefighter mission requires you to use a Firetruck to put out fires. As you delve deeper into the levels, you will not only have to put car fires out, but also people as well that get out of the burning vehicles. As with the other vehicle missions, you do need to complete all 12 levels in a single sitting. There is no perfect place to do this, as with something like the Paramedic one, but you want to look at a more flat area, since the Firetruck is very prone to flipping over on tight turns. Your reward from completing all 12 levels is becoming fireproof, although explosions will still hurt you.