[endgame]

I can't speak to Firefox directly, but I'll say that I almost exclusively use the NewPosts tab to the right of Calendar on the main menu bar, then click the last post arrow in that thread. That thread title becomes greyed out after I've read the new posts in it even after I hit NewPosts again until next session. I'm sure someone who uses your browser will help more, but you may try that option if you haven't used it in the past.