Version control systems can be highly useful tools.
At Mozilla, we've made numerous improvements and customizations to our version control tools. We have custom hooks that run on the server. We have a custom skin for Mercurial's web interface. Mozillians have written a handful of Mercurial extensions to aid with common developer tasks, such as pushing to try, interacting with Bugzilla, making mq more useful, and more.
These have all come into existence in an organic manner, one after the other. Individuals have seen an itch and scratched it. Good for them. Good for Mozilla.
Unfortunately, the collection of amassed tools has become quite large. They have become difficult to discover and keep up to date. The consistency in quality and style between the tools varies. Each tool has separate processes for updating and changing.
I contacted the maintainers of the popular version control tools at Mozilla with a simple proposal: let's maintain all our tools under one repo. This would allow us to increase cohesion, share code, maintain a high quality bar, share best practices, etc. There were no major objections, so we now have a unified repository containing our version control tools!
Currently, we only have a few Mercurial extensions in there. A goal is to accumulate as much of the existing Mercurial infrastructure into that repository as possible. Client code. Server code. All of the code. I want developers to be able to install the same hooks on their clients as what's running on the server: why should your local repo let you commit something that the server will reject? I want developers to be able to reasonably reproduce Mozilla's canonical version control server configuration locally. That way, you can test things locally with a high confidence that your changes will work the same way on production. This allows deployments to move faster and with less friction.
The immediate emphasis will be on moving extensions into this repo and deprecating the old homes on user repositories. Over time, we'll move into consolidating server code and getting hg.mozilla.org and git.mozilla.org to use this repository. But that's a lower priority: the most important goal right now is to make it easier and friendlier for people to run productivity-enhancing tools.
So, if you see your Mercurial extensions alerting you that they've been moved to a new repository, now you know what's going on.