It's most likely stored in the cache itself and to remove those email in question you have to clear the cache and possible history as well to remove those listing.

OK I cleared cache. Where do I go to add an email address that will fill out complete when I add first few characters to that address? Where are current emails stored where I type in first few characters of address that is stored completes by clicking on it?

They are most likely encrypted and stored in the profile folder for firefox. You have to re-enter the email and send in order to have it saved cache for the next time, so when you enter that same email it should fill in the email address.

And did you setup the AutoFill/Save options yet in order to saved any logins or sites that auto fill as needed.

https://support.mozilla.org/en-US/kb/autofill-logins-firefox

I stumbled on what to do. Went to upper right of fire fox 108.0.2 and clicked Settings. took me to general then clicked on privacy security where lower down on page was forms and auto fill I typed in one of my email addresses and next time I get pop up asking me to log into that email a popup appear and show complete email address I can click on to fill in box.