No such add-on exists, as far as I am aware. But just a thought I got randomly. For larger conversations, the subthreaded comment layout of media aggregator sites such as Poal is more easily navigable than the linear threads of forums (phpBB, vBulletin, Discourse, ~~XenFori~~ *edit(typo):*XenForo) and microblogging sites such as Twitter and Gab (that often just show one subreply per reply).