when tabbing windows, drag & drop stops working inside a single window. To reproduce tab a Thunderbird window toggether with Firefox or some other application and try dragging an email to another folder. Untab the windows, try again and it should work.
I'm not expecting this to make a difference, but would mind re-confirming with fluxbox-1.3.3, just for completeness?
Please retest with 1.3.5