You just delete it.

Start typing whatever it is. The suggestions show up instead of clicking on a selection to use it just let it highlight and then hit the delete keyboard key.

• see Control whether Firefox automatically fills in forms_deleting-individual-form-entries

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.

You can remove all data stored in Firefox from a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History > Show All History" or "View > Sidebar > History") or via the about:permissions page.

Using "Forget About This Site" will remove all data stored in Firefox from that domain like bookmarks, cookies, passwords, cache, history, and exceptions, so be cautious and if you have a password or other data from that domain that you do not want to lose then make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of the involved files.

It doesn't have any lasting effect, so if you revisit such a 'forgotten' website then data from that website will be saved once again.

One tip on deleting autocomplete entries: when you arrow down to a selection and delete it, the bad URL stays in the address bar. Arrow to the correct URL before pressing Enter.

Thanks, that was the easiest solution. I had to do it a couple of times. now when I type fir the correct url comes up. I appreciate your help.

Thanks, this is a more complete solution and complements the others I got. It will be more helpful in the long run.