How come females can be firefighters?

Because most firefighters won't have to go into a building and if they do you can be sure the people waiting outside to go in and get them will be men who can carry them back out (the old "2 in, 2 out" rule). Firefighters do more than running into burning buildings... checking on systems of public buildings, car accidents, situations where people need first aid and EMTs aren't there yet, gas leaks, downed lines and trees, etc. There are plenty of jobs a female could do with those that don't involve trying to lift someone out.