Mozilla still uses Mercurial for Firefox development [0]. They're in the process of moving to git and GitHub [1]. I don't know the status of the move, but I contributed a bit to Firefox's build system via Mercurial, and I am glad to see they're moving to git since it's the industry standard. Mercurial felt so alien when I was using it. Git has its own share of UX problems, but finding support for Mercurial was much harder for me, precisely because nobody really uses it anymore.References:
0: https://hg-edge.mozilla.org/
1: https://groups.google.com/a/mozilla.org/g/firefox-dev/c/Qnfy...
arp242|6 months ago
According to the top comment, hg is synced from git (rather than the other way around), and they basically finished the migration AFAIK, with some compatibility/legacy hg stuff for the time being.