Do you run Ubuntu and build Firefox? If so, please check out a mozilla-central revision as of a few hours ago and run mach bootstrap. Among other things, the bootstrapper now installs a modern version of Mercurial (2.7 at the time I wrote this). Ubuntu currently ships 2.2.2 in 12.10 and 13.04. It's even worse for older releases. Mercurial has evolved a lot over the last few months and releases and I don't think you'll be disappointed after upgrading.

If you don't run Ubuntu, you should still run mach bootstrap periodically to ensure your system has the optimal packages for developing Firefox.