Mozilla being Mozilla, I’d guess. They should have gone sel-hosted with sourcehut, or at least gitlab. Or if not self-hosted, the choice should have been at the least gitlab or better, given it allows to chose DCO over CLA. But perhaps not everyone cares… I remember when gitlab introduced DCO, and how that helped debian and gnome to migrate to gitlab. After allowing DCO, other projects migrated as well.
I’m not that fan of gitlab, and I’d prefer sourcehut for open source projects, but if wanting something closer to github, then gitlab might be the answer. But Mozilla is a corp, maybe they don’t care much about these things, and as a corp, perhaps they were looking for CLA sort of contribution any ways…