If you enter Testingcheatsenabled true, then shift click a house, there should be the option to spawn a job, but sadly, I think you have to be at a certain career level to do certain jobs, so if your a level one firefighterl, sorry, you'll probably still be doing small fires, but you can do them a lot more often! Also, if you do this on a community lot, there will be an emergency inside the building, sadly, you don't even get the choice to influence what your sim does. If you want to just be a superhero firefighter, you can also drag the bar that shows how close you are to the next level and just drag it to the top.
Edit: I just went back to the board you posted this on, and, errr, it's in modifications. This is more related to ambitions, maybe you just thought that expansion packs counted as a modification?
