[not OP, but making educated guesses from what has already been said]

Given the post above, issues regarding self-hosting were at least part of the reason for the switch so a new self-hosted arrangement is unlikely to have been considered at all.

I don't know what the state of play is right now, but non-self-hosted GitLab has had some notable performance issues (and, less often IIRC, availability issues) in the past. This would be a concern for a popular project with many contributors, especially one with a codebase as large as Firefox.