All of our feature design discussion happens on mailing lists. Here are the major Firefox development discussion lists:
Design and discussion for new and existing Firefox features and bugs. Join the dev-apps-firefox mailing list then post, or (recommended) use Thunderbird to read and reply through the news.mozilla.org newsgroup: mozilla.dev.apps.firefox.
For discussion of development of the theme used for Firefox, join the dev-themes list. Join the dev-themes mailing list then post, or (recommended) use Thunderbird to read and reply through the news.mozilla.org newsgroup: mozilla.dev.themes.
For discussion of planning for future versions of Firefox, Gecko, and other projects. Join the dev-planning mailing list then post, or (recommended) use Thunderbird to read and reply through the news.mozilla.org newsgroup: mozilla.dev.planning.
There are many lists that contain discussion of other parts of Firefox (e.g. Gecko components) and other Mozilla projects like Thunderbird; as well as general pan-project lists like dev-general and dev-planning: