Yeah, context, especially how you got there. I've been wishing for that for years. Also what your other tabs were doing for any particular tab/bookmark.

I leave a lot of tabs open too, all related to each other. There are usually two or three groups of related tabs. TreeStyleTab addon helps a lot with that, since the vertical tab list shows parent/child tab relationships and lets me move things in and out of hierarchies.

I don't even use FF's bookmarks anymore, which is a shame because I miss the address bar integration. I just use pinboard, which is basic but works.