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.