The issue seems to be very much similar to #18838 which is in closed status now.
THIS ISSUE HAPPENS ONLY IN LATEST FIREFOX VERSION 60.0 (may be earlier ones as well)
On drag and drop from a tree to a Multiselectlist(or any other component) over two different widgets, the drop is not happening. Also Avatar is not changing to Green. The issue seems to be with the onMouseMove function in Source.js or its base classes not getting triggered when the mouse button is already pressed (i.e during a drag operation to a eligible target). Chrome does not have this issue. Any insights on this is greatly appreciated.