mozilla-central is a Mercurial repository of the Mozilla code: https://hg.mozilla.org/mozilla-central .
See How do I check stuff in? for help with hg commit and hg push. If you believe you should have access to mozilla-central but don't know your login details, file a bug.
Ensure your checkins follow the general checkin rules. Occasionally, e.g. just before branch points, mozilla-central also has special tree rules. The Tinderbox page for mozilla-central is located at http://tinderbox.mozilla.org/showbui...i?tree=Firefox.