I was in the middle of asking this question when I fixed (worked around) the problem so I'm posting more as a informative post as there is a lot of old misdirection out there now. I am(was) unable to drag any URLs from the URI drop down field in the main interface to my bookmarks toolbar. Nor was any drag and drop functionality working in the "Customise Firefox" screen (right click space on url bar -> Customise Toolbar). I have now found that resetting my zoom setting in preferences to 100% restores this functionality. The problem arose when I was using 120% zoom, NOT text only.