Would you get fired if you didn't show up to NFL practice every day?

They allow a certain number of missed practices, but mostly they expect you to be at practice and working out. As much as you think you know, they also need to protect you from injury as well as monitor your workout. Remember, the players are not employees, they are investments which must be protected.

In addition, the workouts include practicing plays CONSTANTLY. This is more than just 'running that play til you get it right'. It's a form that must be constantly run and RE-RUN so that the 'muscle memory' comes into play in the middle of a game. Perfecting form is more important than just "knowing the plays."

There is no way to practice execution by yourself.

Sure.

I am pretty sure marshawn skipped a few practices and nothing happened to him lol

Yes, you're required by contract to show up, work out, and drill with the team.

If an NFL player does not show up to practice every day, he/she runs the risk of getting released by the team. Remember, it is important for an NFL player to work hard and perform very well in practice every day. That is the only way he/she can prove that he/she belongs on the roster.

If you don't show up and do the work, odds are you'll get cut. Unless, of course, you're a superstar. But if you don't do the work odds are you won't be a superstar.
When players want to get out practice all they need to do is fake an injury. That happens a lot.

Yes you would, or at least suspended.

Fired, no
Suspended without pay yes

You get fined for missing practice, and no players have other jobs (that would require missing football)