Steps: 1. Make Firefox lose focus to another application. 2. Hover over a bookmarks toolbar item Result: hover effect (indicating click-through) 3. Middle-click Result: nothing happens Expected: open a in a new tab, and maybe focus the window Related to bug 460782? This is one of the cases mentioned in bug 581876.
there is also bug 562998 that could be related
Looks like bug 562998 fixed this.