If you were previously using Mercurial 1.9 in MozillaBuild, I think
you'll find that 2.5.4 is much, much better. I urge everyone to upgrade
for this reason alone.
MozillaBuild was AFAIK the last thing holding us back from requiring
Python 2.7.3 to build the tree. So, I filed bug 870420 to up the build
requirements from 2.7.0+ to 2.7.3+. For the unaware, 2.7.3 fixes a whole
host of small bugs that we constantly have to work around. I'd rather we
just bump the version requirement than continue to toil with workarounds
to fixed Python bugs.