https://support.mozilla.org/kb/automatic-account-configuration

For an existing install, you can navigate to the setup screen via ≡ > New Account > Existing Email.

Existing E-mail Setup

Please, how do I even get to the screen to add another E-Mail? ie: Where is "New Account" and ">Existing Email".......I don't see a "setup screen" where is the "setup screen"? I am 81 years old and need some patient and thorough instructions.....hopefully not to long.....the simpler the better. Thank you. Bryan Baker.

Click the hamburger button ≡ at the upper right of the main window and follow the instructions.