If I'm a doctor, is it legal to waive co-pays for police and firefighters out of the generosity of our heart?

Yes, as long as 1) your contract with the insurer(s), or Medicare or Medicaid, does not require you to accept the copays (if it does, you have to honor or obey the contract terms); 2) you do not try to pass the additional cost on to the insurer, but instead absorb the loss yourself.

Apart from the above, the co-pays are for your benefit primarily (well, that, and to encourage people to be more circumspect about consumming health care, by forcing them to go out of pocket)--if you choose to waive that benefit and give  up the money, that is your choice, and a rather admirable one at that.