How to remove typed-in URLs from browser history in Firefox?

As all things accessible via Firefox's default interface seemed to fail, I decided to take the "deep-dive" approach. So I installed the SQLite Manager plugin, opened the places.sqlite file from within my firefox user profile directory (Menü: Database → Connect Database), and checked the moz_places table (select it in the left pane of SQLite manager) via the search function (in the big right pane: Tab Browse and Search, there the "Search" button) for the "offending" url. The corresponding SQL-Statement (for the Execute SQL tab) would have been:

SELECT * FROM moz_places WHERE url LIKE '%readwrite.com%';

For users prefering the graphical way:

^{Screenshot of the search filter (click image to enlarge)}

As it turned out, there was exactly one hit with a full URL, which did not show up when filtering the browser history/bookmarks! So I took the freedom to delete that record, and tried my urlbar again: Yeah, looks like that solved the issue! But why so difficult, Mozilla?

EDIT: Keep in mind that "operating directly at the heart" you might "kill the cat". Be careful modifying Firefox' databases directly -- or you might end up with inconsistent bookmarks or worse! Always cross-check before deleting, for example.

When you've typed an URL into the address bar, and the autocomplete popup is visible, highlight an URL in the list and press Delete. That will stop that URL being suggested again for any query. If there are many similar URLs you want to get rid of, you might need to backspace the last character and type it again to refresh the list.

Note: This doesn't work on bookmarked URLs. Hitting Delete removes them from the list while it's open, but the bookmark will be suggested again next time you type a similar URL. If that causes you a problem: on the Options dialog, on the Privacy tab, there's an option to make it only suggest from history, not bookmarks.

There's also another kind of autocomplete, where a domain name appears inline in the address bar itself (not in the autocomplete list) as you type. It only completes the domain name, not the whole URL, so it doesn't cause the "too many completions for stackoverflow.com" problem you describe.

The suggestions come from the same list, so if you delete all the autocomplete items for a domain and you don't have any bookmarks for that domain, it won't suggest that domain inline either. This can be quite tricky to arrange, as in your example, if you type "rea" and it completes readwrite.com but the list is full of pages about "reanimation". In that case, you need to let it complete readwrite.com by pressing End (not Enter), and then backspace the last character to refresh the list. Then only the readwrite.com entries will show in the list, and you can delete them in the same way. Again, if there are lots of entries, you might need to backspace the m to refresh the list.

You can disable just the inline-autocomplete feature by going to about:config and setting browser.urlbar.autoFill to false. That setting doesn't affect the drop-down list of suggestions, only the inline suggestion.