When you visit about:foo,
moz_origins ends up with a row where prefix="about" and host="foo". That's correct insofar as the scheme is "about" and the "host" is "foo".
The erroneous trailing slash is due to the fact that the origins query assumes all URIs have paths, in these two lines: https://searchfox.org/mozilla-central/rev/97cb0a90bd053de87cd1ab7646d5565809166bb1/toolkit/components/places/UnifiedComplete.jsm#175 When I remove those trailing slashes, about:config is actually autofilled correctly when I start typing "config".
So that's one problem.
The other problem is that we probably should not be autofilling "config" in the first place. We could either add a migration to remove all "about" URIs from
moz_origins combined with stopping adding them in the first place, or we could just modify the query to exclude them.