Well arguably the long-term result for Mozilla is that we have ended up having to support two version control systems for everything because there has been less

Well arguably the long-term result for Mozilla is that we have ended up having to support two version control systems for everything because there has been less resistance to that than trying to force people to use mercurial for everything (for the projects that were already in mercurial people wrote git bridges, and for new projects people tended to start them on GitHub). I strongly doubt that there would have been the same problem if we had used git from the start. I suspect this is a net negative, even if you believe the rhetoric that mercurial (or future-mercurial) has technical advantages in some areas. Edit: to be clear, the kind of "everything" I have in mind is the code review system which now has to support two version control tools. Indeed if we'd used git from the start there might well have been an off-the-shelf code review tool we could have used without putting multiple man years of effort into customisations.