The Xen Project chose Mercurial to replace Bitkeeper (the proprietary but free-as-in-beer-to-open-source-projects-for-a-while VCS that Linux used before it became no-longer-free-as-in-beer) as well. There were things that were nicer about it; but in the end it just made more sense to move over to git.

That was over a decade ago, however; I'm actually pretty surprised it took Mozilla so long to switch.