The ‘External maps’ panel is scrollable in Firefox 60:

1. Go to https://ru.wikipedia.org/wiki/Казань#/maplink/0
2. Press ‘Внешние карты’ in the bottom if menu hasn’t opened itself
3. See scrollbar on the right and scrollbar in the bottom

It was intentional, as I understand, to have scroll on the right side as not all information could potentially end up on the screen, but having scrollbar on the bottom is a side effect of using overflow-y: scroll; without a declaration for overflow-x.

Screenshot:

Way to fix:

.mw-kartographer-mapDialog-sidebar {
   overflow-x: hidden;
}

Or you could fix issues that cause having the overflow in the first place, but that is more complicated.