Firefox doesn't have anything to do with AOL email. It's just the browser to open your AOL email. You have to look at the FAQ in AOL if they have one listed to see how to do a signature for AOL email.

Hi Marilyn, are you using AOL inside of Firefox (on AOL's website) or in a separate mail program like Mozilla Thunderbird or Microsoft Mail? Each site and mail program will have its own different method for creating and inserting a signature.