On the click-vs-hover issue, I actually very much like Firefox's approach, and wish it were more widely used.
W.r.t. it being an "old, solved HCI problem", it isn't. The link you post actually posits positioning solutions to problems inherent to the hover approach. It doesn't discuss hover-vs-click at all, and while these positioning strategies still apply to some degree with the click approach, the problems they solve are much less severe there (namely, the submenu doesn't disappear if the user takes the wrong cursor route to a submenu item).
W.r.t. it being an "old, solved HCI problem", it isn't. The link you post actually posits positioning solutions to problems inherent to the hover approach. It doesn't discuss hover-vs-click at all, and while these positioning strategies still apply to some degree with the click approach, the problems they solve are much less severe there (namely, the submenu doesn't disappear if the user takes the wrong cursor route to a submenu item).