I'm on F.M.L.A. and I have been fired from my job.

You should immediately contact an attorney as it appears there was a violation of the FMLA.  The FMLA forbids retaliation for seeking FMLA.  The termination also interfered with your ability to take FMLA.  The company could terminate you only if they can prove that the termination had nothing to do with your FMLA request.  For example, if the company planned a lay off for several employees and would have been laid you off regardless of whether you requested FMLA or not, you may not be protected.  You should meet with an attorney who can review your case in more detail