I prefer the "hckr news" extension. In addition to collapse/expand, it has a "[collapse whole thread]' option so that you can do it in the middle of a thread,

I prefer the "hckr news" extension. In addition to collapse/expand, it has a "[collapse whole thread]' option so that you can do it in the middle of a thread, and most importantly, puts a vertical orange bar to the right of all new messages since I last refreshed or visited the page.