What steps will reproduce the problem?
======================================
The problem can easily be reproduced on the public page http://www.w3schools.com/html/html5_draganddrop.asp.
For instance, using the Python bindings:

from selenium.webdriver import Firefox
from selenium.webdriver.common.action_chains import ActionChains
ff = Firefox()
ff.get("http://www.w3schools.com/html/html5_draganddrop.asp")
source = ff.find_element_by_id('drag1')
target = ff.find_element_by_id('div2')
ActionChains(ff).drag_and_drop(source, target).perform()

What is the expected output? What do you see instead?
=====================================================
I would expect the "w3schools.com" image to be dragged and dropped onto the right div
(/box).
Instead, the last command ActionChains(..)... blocks until I move my mouse cursor over
the Firefox window. Then, the command returns but the image was not dragged.

This also happens when native events are enabled:

from selenium.webdriver import Firefox, FirefoxProfile
from selenium.webdriver.common.action_chains import ActionChains
profile = FirefoxProfile()
profile.native_events_enabled = True
ff = Firefox(profile)
ff.get("http://www.w3schools.com/html/html5_draganddrop.asp")
source = ff.find_element_by_id('drag1')
target = ff.find_element_by_id('div2')
ActionChains(ff).drag_and_drop(source, target).perform()

Selenium version: Python-2.35.0
OS: 64 bit Windows 7
Browser: Firefox
Browser version: 23.0.1

Many other users seem to be having the same problem:
 * http://stackoverflow.com/questions/18059684/issue-with-drag-and-drop-element-in-selenium-web-driver-with-java
 * http://stackoverflow.com/questions/16081075/unable-to-perform-drag-and-drop-using-selenium-webdriver
 * http://stackoverflow.com/questions/12117640/selenium-webdriver-not-working-for-html5-drag-and-drop
 * http://stackoverflow.com/questions/9797462/html5-drag-and-drop-using-selenium-webdriver-for-ruby

A fix for this would be greatly appreciated. I tried to work around the problem using
JavaScript, but to no avail.