Since I'd have no other superpowers, the idea of destroying a few terrorist cells or the mafia of an entire country wouldn't be feasible. Therefore, try to do something intelligent. If criminals can't stay undercover, they're more likely to get caught. So, a good thing to do would be to show up (with some disguise) in places where criminals are likely to hang out, and tick them off enough to get them to shoot me. When they see it has no effect, they'll freak out.
Done several times in various places on various days, this should freak out a good number of criminals and probably have the word spread throughout the criminal community. Hopefully, that would lead to them becoming reckless, less careful, more scared, etc., in turn leading to a large number of criminals being apprehended by the authorities. With only temporary invincibility, I can't think of any more effective to deal with crime.
Other than that, I really like the firefighting idea. You could save many lives in a month that way.
That all is assuming you also don't feel pain. Feeling the pain from being shot multiple times or from burning without dying doesn't strike me as a good choice.