Can I fire my public defender at my plea hearing?

You can always "fire" your attorney at any stage of a criminal proceeding, however when a defendant decides that they are going to fire their criminal defense attorney, the Judge will often have you state before him or her the reasons why you wish to have a new attorney on your case, and it will generally be up to the Judge to determine if you will be allowed to do so or not. Although it is your attorney who generally negotiates a plea deal on your behalf, the firing of your attorney does not in itself revoke or terminate any existing plea deals. My advice is to either contact your current attorney and inform them of your intentions, or contact a local criminal defense attorney in your area for further specifics of how your judge will handle your situation.