Since the feature hasn't been implemented yet, and nobody else has offered a workaround, here's how you can manually remove a single website (or multiple specific websites) from your browsing history on Firefox for Android. You need to be using a rooted phone, as otherwise your phone vendor is blocking you from accessing and managing your own data.
$ su
# sqlite3 /data/data/org.mozilla.firefox/files/places.sqlite
> DELETE FROM moz_places WHERE url LIKE '%whatever%';
> DELETE FROM moz_origins WHERE host LIKE '%whatever%';