I had read this twice and still have to as what are you going to sue them for?

It is not a crime to be rude nor fire someone without notice and you have no actual hard proof that it was retalitory. I mean yes it seems it was but this was all verbal, comes down to a he said she said. You even said new owners bought one of the locations, you have no idea what is or is not going on behind the scenes.

Why did they sell? Are they going to sell the second location in the near future? Are they cutting costs? Are they rebranding? Do they maybe just not like you anymore? Anyway outside of you telling a lawyer you were fired due to bringing this up, there is no actual proof and he will have a hard time proving it, employers by law do not have to give any reason or explanation when someone is terminated.

"Employers are generally not legally required to explain to an employee why he or she is being discharged. There are no federal laws that require employers to explain to the worker their decision to fire the employee"