I don't really have any ideas here—best as I can recall we've never seen this. If you can provide a screencast (using Jing or something similar, maybe), we might be able to notice something, but at this point it seems like this is something unique to your system setup.
it might be, do you have any guess what might it be? as I mentioned again and again, with firefox 3.6, everything works as expected on the same computer. can this be a clue?
If I drag the object from the link in a web page, then it works. I still cannot drag from the address window.
I can also drag a pdf from outside firefox now.
Puzzling. I am not sure if the system is somewhat upgdated automatically.