Created attachment 626338 [details]
screen shot of the panel with no anchor and no right border
If I move the FF window to very near the right-hand edge of the screen and click on one of the notification icons, the panel is displayed without an anchor (ie, there is no "arrow" pointing at the button). If FF is moved away from the right hand side of the screen it correctly appears. Attaching screen-shot.
I've seen this in various situations, it seems like a platform bug, I haven't been able to figure out how to fix it. Most often, on OSX, it is the first time a panel is opened, after which it behaves correctly. In my windows VM, it seems to happen more often.
The fix for bug 757746 seems to have fixed this as well, verified on windows and osx. The problem I mentioned before had to do with panels showing the anchor on a side rather than top.