Some lists remain active and have moved elsewhere. The google group lists can be subscribed to via the links below or by sending email to the address +subscribe@mozilla.org
announce @ mozilla.org - product releases and other announcements
dev-media @ mozilla.org - For discussion around media in browsers (Video, Audio, WebRTC, etc.)
dev-platform @ mozilla.org - discussions around web platform development (gecko)
dev-security-policy @ mozilla.org - security-related policies, governance, and related topics; including discussion of Mozilla's Root Store Policy and the NSS root certificate store.
dev-tech-crypto @ mozilla.org - mozilla's crypto code discussion list
dev-tech-gfx @ mozilla.org - Gecko platform graphics mailing list
dev-tech-network @ lists.mozilla.org - Mozilla's network layer
enterprise @ mozilla.org - development of features and management of Mozilla products like Firefox for Enterprise environments
governance @ mozilla.org - Discussion of the governance and running of the Mozilla project
nightly-testers @ mozilla.org - discussion of features and behavior changes in nightly builds
sync-dev @ mozilla.org - Firefox sync development discussions
Thunderbird lists have moved to topicbox