Firefox doesn't "do email", it allows you to view and use "web mail" on the internet. Have you tried using the Zoom controls inside Firefox? The basic zoom controls for the "web mail" interface are in the software that they use, so check their support documentation at that website to learn how to change the basic user settings there, or how their keyboard zoom controls work, to see if that will work. Or if "we" knew which web mail portal you (like Gmail, Yahoo!Mail, or other) we should be able to help you more.