For a long time Firefox Desktop development has supported both Mercurial and Git users. This dual SCM requirement places a significant burden on teams which are already stretched thin in parts. We have made the decision to move Firefox development to Git....
At least when it comes to Git I'm not too concerned. What could MS possibly do to you? Maybe vendor lock in via the issue tracker? They aren't using it and it's not exactly that hard to migrate off of it in the first place.
Firefox Development Is Moving From Mercurial To Git (groups.google.com)
For a long time Firefox Desktop development has supported both Mercurial and Git users. This dual SCM requirement places a significant burden on teams which are already stretched thin in parts. We have made the decision to move Firefox development to Git....