They can't fire you for being average. Only below average.
Most definitely not true.
IIRC, Missouri is a state covered by much the same employment laws (as respects hiring and firing anyway) as Illinois. I believe they're called "at-will" states. You remain at your job at both your will and your employer's will. You can be fired without notice, and you can quit to go work for another company without notice (two weeks is NOT required; merely customary and usual.)
And, believe me, you can get fired in Illinois even if you're a great employee. Employers don't need any reason to fire you.