I've been using git for a few years for doing Mozilla work, which has caused me to have to go through a painful workflow every single time that I push code to inbound. Now, glandium has written this awesome tool that finally enables me to use git push to push an actual set of git commits to the hg servers, directly, without running Mercurial. My existing workflow for pushing involves exporting textual diffs from git, and importing the file into Mercurial, and deal with the conflicts myself, etc. I often get it wrong and end up pushing half-patches that don't compile. Even when that doesn't happen, I find the interaction with Mercurial quite draining. Mercurial is so much slower in very common every day operations and sitting there and waiting for it makes me restless. If any of these resonates with you, glandium has written a blog post which can help you get started. Please go and read it.
You need to do the initial clone through git-remote-hg. If you already have a clone of gecko-dev, you can safely fetch hg:: remotes inside there. For example, you can fetch hg::ssh://hg.mozilla.org/integration/mozilla-inbound for rebasing and pushing to inbound through there. To make things easier, you can gradually move your work based on branches from git-remote-hg and slowly migrate away from the gecko-dev based branches.
One interesting side-note: I measured the amount of storage that my local clone's .git directory takes versus that of an hg mozilla-central clone. Note that my local git repo contains both the entire gecko-dev history (which includes the CVS history) and the repo created by git-remote-hg. Its size is 2GB, versus an 1.8GB .hg directory of just mozilla-central. Given that I have accumulated tons of local hg clones from all of our central and release and project branches, I'm really looking forward to the disk space saving when I finally decide to delete most (all?) of my local hg clones! :-)