You can use these steps to remove saved (form) data from a drop-down list:

1. click the (empty) input field on the web page to open the drop-down list
2. highlight an entry in the drop-down list
3. press the Delete key (on Mac: Shift+Delete) to remove it.

See also:

• Firefox > Preferences > Privacy > Firefox will: "Use custom settings for history": "Remember search and form history"
• https://support.mozilla.org/kb/Form+autocomplete

It didn't work. I read the 'see also' suggestions. It said the same thing but shift delete over the popped up name did nothing.

This is a problem with an input field on a website?

If it is somewhere else like in the Firefox user interface like the search bar on the Navigation Toolbar then please clarify.

Note that some web pages may show suggestions in the drop-down list and that you aren't seeing names that you actually typed yourself.

This is within the Thunderbird app, not on the Firefox browser. When I do a search on my mailboxes, I get everyone on my computer. I want to be able to clean that out and delete some of the names, without deleting everyone. How can I do that.

I don't think you can delete results of the autocomplete from the To: box. Since autocomplete searches all address books for matches, the ones you want to eliminate are probably in Collected Addresses, so you would have to delete the obsolete contacts from there, or any other address book that contains them.

There is an add-on that, among other things, allows you to specify the address books that are searched by the autocomplete routine:

https://freeshell.de//~kaosmos/morecols-en.html

so you could, e.g., restrict searches to Personal Address Book or a separate 'Speed-dial' address book.

Thank you for that added dimension.

Where is "collected addresses" does that exist for me to click on?

If you open Address Book, all your address books, including Collected Addresses, should be visible in the left pane. If the pane is hidden, select View/Layout/Directory Pane; press Alt if the menu Bar is hidden.