I am astonished that this issue, reported over 5 years ago in ticket 3740 is still occurring in v.1.10.3. And, the script modification that fixed it in 3740 is still fixing it when added to v.1.10.3.
In my case, I have some tables designed to look like list boxes, which scroll through a list of text entries. These text entries are dragged from one box to another. It all functions correctly in Chrome, but breaks as previously described when the list is scrolled in FireFox. To reiterate, this involves a scrolling document containing tables which scroll text.
A small amount of scrolling in the list boxes results in a 300px to 400px offset between the cursor and the helper clone. If anyone is interested, I can grant access to this Web page, so you can experiment with it.