Mailing Lists
Mozilla maintains a great number of discussion spaces for its projects. A mostly-complete list is available at https://www.mozilla.org/en-US/about/forums/ , which is organized roughly by priority. Most of these consist of a Google Group, a Mailman mailing list hosted on https://lists.mozilla.org/, and an NNTP newsgroup, all of which are joined so that a message on one gets cross-posted to the others. This way, you can follow along using whatever mechanism you're most comfortable.
A very few mailing lists are also maintained on https://mail.mozilla.org/. These mailing lists are not necessarily mirrored to Google Groups or newsgroups. Some mailing lists are set up this way to allow for the possibility of moderating messages before they get sent out, which is not possible on an NNTP newsgroup.
There are also some additional mailing lists that are not set up in this way, but are hosted externally to Mozilla.