The Mozilla Newsgroups (also availalble as mailing lists)
http://developer.mozilla.org/en/docs/Newsgroup_summaries http://wiki.mozilla.org/MDC_newsgroup_summaries
Groups and number of new topics (i.e., not individual messages) of messages since January 2006
mozilla.dev.apps.firefox 404 mozilla.dev.l10n 317 mozilla.dev.tech.js-engine 247 mozilla.dev.planning 224 mozilla.dev.builds 181 mozilla.dev.apps.thunderbird 163 mozilla.dev.tech.layout 76 mozilla.dev.quality 59 mozilla.dev.security 29 mozilla.dev.i18n 14