Dragging items in Tree component is sluggish in Firefox

In the Angular component 'Tree', dragging items is sluggish in Firefox. It works fine in Chrome and IE10.

Steps to reproduce:
1. open the Hippo Theme demo in Firefox (tested in version 33)
2. open the 'Angular Components'
3. drag an item to another position in the tree

Expected: the dragged item moves along smoothly with the mouse pointer, snapping into new positions as a mouse pointer moves
Actual: the dragged item moves along with the mouse pointer, but very sluggish. Only when dragging very slowly the item snaps into place.

This affects the usability of the Tree component quite a lot.