I can't believe this annoyed me for soooo long.
My library allows managing our library accounts over the internet. It requires entering our library card number as a user name and our date of birth as the "password".
Over the years, I made some typing mistakes, like accidently typing in the current year instead of the actual birth year. As a result, this wrong entry stuck, forever for years, anyway. Now, when you start typing, you get a dropdown, but it's not as easy to just pick the correct entry any more are there is so much junk in it.
It annoyed me so much that I went looking for a Firefox Extension to manage autocomplete entries for me. And then, I found this page which showed me I don't even need an extension: it's part of the Firefox core! Yes, they talk about the URL bar, but it does work on plain text input fields.
All you have to do is make the dropdown appear, move the highlight bar over the entry you want to get rid of, and press shift-delete. Pooff, it's gone!