Modifying Mozilla code on a separate Hg repo?

Mercurial scares me, simply because I've never worked with it before. 
Yet I sense instinctively that it will be easier to work with a pure-Hg 
solution and to modify copies of the cvs-trunk-mirror (1.9-base) 
repository... hosted on locations other than hg.mozilla.org.

Here's the scenario I envision, and I hope someone will tell me how I 
can do all these:

* I want to host a copy of the cvs-trunk-mirror repository on mozdev (or 
at Skyfire, behind an internal firewall), and check out from that 
location instead of from hg.mozilla.org.  Then I would make changes to 
that repository instead of the official cvs-trunk-mirror.  (We do modify 
  native Mozilla code a bit to meet our needs.)

* mozilla.org does updates to cvs-trunk-mirror for 1.9.0.x releases, 
which I want to import as a batch into the private repository.

I'm looking for a hg-only solution instead of a hybrid cvs+svn solution 
which I've been doing for several months now, which is a pain in the 
butt to maintain already.  Any directions you could provide would be 
welcome.

Alex Vincent
Skyfire Labs, Inc.