Yes, I’m using Firefox too and I’ve noticed the same effect. The pop up displays in one position or other depending on where the SPOT [+] is placed at that very moment in the screen.
See following sceenshots showing the pop up displayed in different ways depending on the scroll position of the SPOT [+]:
When the SPOT [+] is around the central part of the screen, the pop up window is displayed at the side and it’s fully visible as long as the number of grouped spots is not too big, I guess:
When the SPOT [+] is at the top of the screen, the pop up window is at the top and we can’t see all the spots:
In my opinion, this would be the perfect way for this to be displayed:
At the side and being the top limit of the pop up window always at the same level of the SPOT [+] row.
That’s the way all the grouped spots will always be visible even when the SPOT [+] is placed at the very top of the screen.
In case it is at the bottom of the screen, we will have to scroll up a bit to place the SPOT [+] towards the center of the screen but that’s something we can always do easily, but we cannot scroll down when the SPOT [+] is at the top of the spots list.