Are they losing contributors, though? I like GitHub/Gitlab/BitBucket as much as most people but choice of version control is going to be secondary to getting your patch into a major web browser in my opinion.

Also looks like they're still planning on using existing review tools:

> We will continue to use Bugzilla, moz-phab, Phabricator, and Lando... Although we'll be hosting the repository on GitHub, our contribution workflow will remain unchanged and we will not be accepting Pull Requests at this time