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Firefox Development Is Moving From Mercurial To Git
(groups.google.com)
A place to discuss the news and latest developments on the open-source browser Firefox
So it's not really moving from one to the other, it's already on both and they're closing the less popular one.
No. The primary source control is on Mercurial, with a Git interface. But since all patches must go through moz-phab (Phabricator) anyway the Git interface is basically read-only – you can use it to pull the code and you can use Git locally for your development, but you can't use Git to push back into the main repo.
Now the reference repository will switch from their own Mercurial infrastructure to a Github (Git) repository, but the flow will remain the same (Phabricator rather than GitHub pull requests). Nothing will change for devs who were using Git. Devs who were using Mercurial will have to switch to Git.
why didn't they go to Gitlab though? now they will be hosted by microsoft
the github repo is a mirror to their internal source hosting. github is just the most accessible to a wide audience