Depends a bit on what the default cloning url will be. If the domain is in control of mozilla, which forwards it to github, then fine, if most people start using the github url, then it is still a vendor lock in, because many people and projects will use it, and that is not so easy to move away.

Update: To the people down-voting my comment, I would love to hear why you either disagree with me, or find that my that my contribution to this discussion is worthless.

The upstream URL of a project or repo is important, because it will be used in other projects, like in build scripts for fetching the sources. If a projects changes that URL in the future, and the old URL is no longer available/functional, all those scripts need to be changed and the old versions of these scripts do not work anymore out of the box.

If the project owns the URL, then can add redirect rules, that might help alleviate some of these issues. I don’t think github allows projects that move away from it to do that. So this is a sort of vendor lock-in. The project needs to maintain the repo on github, because they want to break the internet as little as possible.