You can set it up as follows:
git clone http://people.mozilla.org/~jmuizelaar/mozilla-cvs-history.git
git clone git://bluishcoder.co.nz/git/mozilla-central.git
# set up symbol links to cvs-history pack files
ln -s ../../../../mozilla-cvs-history/.git/objects/pack/pack-5b5d604ab48cf7bc2a6b4495292fa8700a987c5f.pack .
ln -s ../../../../mozilla-cvs-history/.git/objects/pack/pack-5b5d604ab48cf7bc2a6b4495292fa8700a987c5f.idx .
# add a graft from the last revision in the mozilla-central repo
# to the first revision in the cvs-history
echo 2514a423aca5d1273a842918589e44038d046a51 3229d5d8b7f8376cfb7936e7be810635a14a486b > info/grafts
Now you have a git repository containing all of the history. You can update the mozilla-central repository as you normally would. The conversion isn't perfect, but it's been good enough to have working blame back into cvs time.