Why GitHub? Of course it's the most popular (due to goodwill before it was acquired, and then inertia), but there are also other options that might be a better match for Mozilla's open mission.
A bit sad (I prefer hg.) But at least just about everyone is now using a VCS, which was very far from the case when I started programming seriously back in the early 1980s.
I don't blame them but I do feel there's a certain irony at work. The same forces that lead us in the direction of a browser mono-culture also lead us towards a source control mono-culture!