Getting fired: Can you? Yes. Can you lose your job without being fired? Also, yes.
"WTF did you just write, A-dub?"
Here's the thing, a company does not make money unless it has a body in a slot. Take your salary, double it AT A BARE MINIMUM, and that's what your company is billing the gov't for your services. Companies have a very vested interest in keeping people on contract. That means...you guessed it, you are like the Mob: blood in, blood out. Getting fired is uncommon, you typically won't have your contract renewed if you're a turd. With that said, I know or know of people who were fired for: porn on a gov't computer, forging certifications, stealing mail, stealing equipment, running a brothel at Bagram, battery, drug/ alcohol possession, and shockingly... general incompetence.
If you make it past your first 90 days (that's the usual period, but it varies) then you are practically golden for the rest of the year. That doesn't mean you should go out and be a turd, but some of your coworkers will stink it up. That is an unfortunate aspect of the business. But...there's another way to be sent home.
Your commander has the right to say, in effect, "I've lost confidence in this person's ability to do their job." They can then just refuse you admittence to their area. So...how can do your job if an O-5 or above doesn't want you in their office space?
Exactly. At that point your company can fire you or transfer you with the latter usually happening. I've seen that 4 or 5 times over the years. Making that sweet bank in Afghanistan, but piss off the wrong person? Now you're going to Qatar for half the money. Congrats!
Remember my earlier post about networking and getting hired? Yeah, don't be a shithead, you might actually need someone to give you an introduction. Consider every single day to be a job interview.