[4] looks like it's only a runner for the actual testing, which is a separate crate: https://github.com/mozilla/memtest
(see: https://github.com/mozilla-firefox/firefox/blob/main/toolkit..., which points to a specific commit in that repo - turns out to be tip of main)
Yes.
I pointed to the runner because otherwise it seemed likely I'd get a comment insisting there's no proof it's used in Firefox.