So when I am filling out webforms in Firefox I can click on a field like " email " and my email is in the options I can choose but another email is there that I don't use. How do I delete the email I do not use from showing up in the form field menu?

I hope you can understand my question . I fill out a form I see a black space in " email " field ... I click on that black field and I can choose what email I want to fill the form with. I want to delete the email there that I do not use. I know on some browsers you can just put the cursor one you want to delete and press delete key but this does not work in Firefox.