A user on the English Wikipedia reports two problems when attempting to drag and drop an image.
"Problem 1: Clicking on a media file and dragging the mouse causes the cursor to change to the "I'm moving an image" icon, but the image doesn't actually move. I can only move an image if I click it once to select it, then click and hold.
Problem 2: If I move an image a small enough distance across the screen that it doesn't actually move, I can't move it again unless I deselect it and then reselect it."
They are using Firefox 21, I can confirm both problems in Firefox 22.