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....
I agree, and GitHub allows choosing how to merge each PR individually if you need to do something different for a specific PR. Large PRs like that are at most 1% of our total PRs, and we review those more per-commit and use a merge commit instead of a squash. By default we optimize for the other 99%.
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....