Suggestions in a drop down list that have a key icon in front are login suggestions that you need to remove in the Lockwise Password Manager. Suggestions in a drop down list without a key symbol are autocomplete saved form data.

• https://support.mozilla.org/en-US/kb/password-manager-remember-delete-edit-logins

Sugestions without a key icon are part of autocomplete form data that you should be able to remove via the cursor down key and Delete (Mac: Shift+Delete).

The problem appears to be that Firefox treats email addresses and logins as the same thing, and when I get a fill-out form, Firefox picks an email address from my login information connected to a search site, and I do not use that login/email address when I am providing an "official" email address on a form requesting email address information. If I delete the "login information" for the search site, that's not good. How do I tell Firefox to use a particular login address as the "normal" email address rather than the one it decided to pick on its own?!

Can you clarify whether the email suggestions have a key icon ?

There is no "key icon".

There is no "key icon".

Let's say you have an email field and you type the first letter. Firefox will show "form history" entries in a little drop-down so you can pop a past entry into this form. To remove a form history entry, press the down arrow key on the keyboard to select it, then press Delete or Shift+Delete to remove it.

Is the problem with that kind of "form history" autofill? You also can turn off this feature completely as a test. See: Control whether Firefox automatically fills in forms.

If there is no key icon then this means that this is normal saved form data and this data isn't stored website specific (might be connected to the name/id of the input field), so that would explain why you see suggestions from emails from other websites.