I have an opinion on the usage of Mercurial at Mozilla: it stinks.
The server is configured poorly
Our Mozilla server, hg.mozilla.org, is currently running Mercurial 2.0.2. In terms of Mercurial features, stability, and performance, we are light years behind.
You know that annoying phases configuration you need to set when pushing to Try? That's because the server isn't new enough to tell the client the same thing the configuration option does. It will be fixed when the server is upgraded to 2.1+.
Furthermore, we are running the server over NFS, which introduces known badness, including slowness.
I believe we blame Mercurial for issues that would go away if we configured the Mercurial server properly.
Fortunately, it appears the upgrade to 2.5 is near and I've heard we're moving from NFS to local disk storage as part of that. This should go a long way to making the server better. The upgrade can't happen soon enough.
User education is poor
I think a lot of people are ignorant on the features and abilities of Mercurial.
I commonly hear people are dissatisfied with the behavior of their Mercurial client. They encounter performance issues, bugs, corruption, etc. Nine times out of ten this is due to running an old Mercurial release. Just last Friday someone on my team asked me about weird behavior involving file case. My first question: what version of Mercurial are you using? He was running 2.0.2. I told him to upgrade to 2.5+. It fixed his problem. If you aren't running Mercurial 2.5 or newer, upgrade immediately.
I've heard people say we should switch to Git because Git has feature X. Most of the time, Mercurial has these features. Unfortunately, people just don't realize it. When I point them at Mercurial's extensions list their eyes light up and they thank me for making their lives easier.
I think a problem is a lot of new Mozilla contributors knew Git before and only pick up the bare essentials of Mercurial that allow them to land patches. They prefer Git because it is familiar and just don't bother to pick up Mercurial. The potential of Mercurial is thus lost on them.
Perhaps we should have a brown bag and/or better documentation on getting the most out of Mercurial?
The branching model is far from ideal
For Gecko/Firefox development, we maintain separate repositories for the trunk and release branches. This introduces all kinds of annoying.
We should not have separate repositories for central, inbound, aurora, beta, release, etc. We should be using some combination of branches and bookmarks and have all the release heads in one repository, just like how the GitHub mirror is configured.
As an experiment, I created a unified Mercurial repository. Each current repository is tracked as a bookmark (there are instructions for reproducing this). Unfortunately, the web interface isn't showing bookmarks (perhaps because the version of Mercurial is too old?), so you'll have to clone the repository to play around. Just run hg bookmarks and e.g. hg up aurora after cloning. Warning: I'm not actively synchronizing this repository, so don't rely on it being up to date.
A Mercurial contributor (who is familiar with Mozilla's development model) suggested we use Mercurial branches for every Gecko release (20, 21, 22, etc). I think this and other uses of branches and bookmarks are ideas worth exploring.
We're failing to harness the extensibility
Gecko/Firefox has a complicated code lifecycle and landing process. This could be significantly streamlined if we fully harnessed and embraced the extensibility of Mercurial. While there are some Mozilla-centric extensions (details in my recent post), I don't think they are well known nor used.
I think Mozilla should embrace the functionality of extensions like these (whether they be for Mercurial, Git, or something else) and invest resources in improving the workflows for all developers. Until these tools are obviously superior and advertised, I believe many developers will unknowingly continue to toil without them. And, it's likely hurting our ability to attract and retain new contributors as well.
Mozilla's current usage of Mercurial is far from ideal. It's no wonder people don't like Mercurial (and why some want to switch to Git).
Fortunately, little has to do with shortcomings of Mercurial itself (at least with newer versions). If you want to know why Mercurial isn't working too well for Gecko/Firefox development, most of the problems are self-inflicted or the solutions reside within each of us. Time will tell if we as a community have the will to address these issues.