[lutrinus]

It's right there in the first paragraph:

> 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.

[sideshowb]

Nitpick, but that quote doesn't explain why not move to mercurial (although I think we all know why)

[pointlessone]

It was Mercurial-only for a long time. Then people got used to git and asked for a mirror to stop dealing with whatever was the layer providing mercurial access for git locally. Apparently, now there are more git people out there than Mercurial.