First, how long have you been there? If you're still in your probationary term, they can fire you for whatever reason they see fit. They do not need to provide any type of reason beyond "It just wsn't working out" during your probationary period.
Second: no offence, but what kind of an employee were you? Were you ever late? Did you ever miss work and not call (or not call until after your shift had started)? Did you call in sick often?
We've fired people while they've been injured, but it had nothing to do with their injuries.
Example: Guy crushes thumb. Guy has shitty attendance record. Guy is often late. Guy has been written up for attendance before. Guy doesn't call into work while on modified duty. Despite being injured, it's still his responsibility to call us and let us know he's not going to be there. Guy is fired. Guy got what he deserved.
Kris
Founding member of the Leave-Me-Alone-atarian party of Canada.