You could still claim prior art if you used or published the idea before they patented it. The difference between "first to invent" and "first to file" basicall

You could still claim prior art if you used or published the idea before they patented it. The difference between "first to invent" and "first to file" basically only comes up when someone invents an idea but keeps it private, which can be difficult to prove anyway. (IANAL, TINLA.)