As of a few minutes ago, we explicitly dropped support for Mercurial 3.0 for all the Mercurial code in the version-control-tools repository. File issues in bug 1162304.

Code may still work against Mercurial 3.0. But it isn't supported and could break hard at any time.

Supporting multiple versions of any software carries with it some cost. The people writing tooling around Mercurial are busy. It is a waste of our time to bend over backwards to support old versions of software that all users should have upgraded from months ago. Still using older Mercurial versions means you aren't getting the best performance and may encounter bugs that have since been fixed.

See the Mozilla tailored Mercurial installation instructions for info on how to upgrade to the latest/greatest Mercurial version.