The is in reference to threads created to discuss a article in the Knowledge Base or KB https://support.mozilla.org/ A example kb article https://support.mozilla.org/en-US/kb/install-firefox-linux you can click on the Discussion in left sidebar (may need to expand the Editing Tools) and there you can start a new thread or comment in a existing one about the article. It is not for getting support but more for say discussion on say changes for current or future Firefox versions that needs to be made in article for example. Every time a new kb discussion thread is made or commented in it is listed at https://support.mozilla.org/en-US/kb/all/discussions