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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.
- 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
- We're still working through the planning stages, but we're expecting at least six months before the migration begins
I think the trouble is when users start committing to git, they can't really be mirrors because each one needs to combine the inputs from both. A mirror would be just a copy of one to the other, this is a constant merging exercise which requires significantly more effort.
Downvotes without explanation as to why they think that doesn't make sense.