What legal form would you use to fire your lawyer?

Unless Texas has some kind of unique law that requires this... in the case of the firing of a private attorney, no form should be necessary. If you are in a middle of a case, the attorney will be responsible for notifying the court that he will no longer be representing your interests. If you are being represented by a Public Defender... then the whole picture may change somewhat, and you may need to submit a form requesting another public defender other than the one you were originally assigned.