FDP can be configured in different ways, so you might need to explain what you mean by "doesn't work" and also if you are using it on a "new" Firefox installation or an old one. Apparently if you upgraded from say FF14 to FF15, things are not broken as badly as if you put FF15 on a new machine.
On my setup, with a pre-existing FF upgraded to 15.0.1 , FDP is only slightly broken. I can drag an image file onto the TW, and it makes a new tiddler. However, the image associated with the tiddler no longer displays automatically.
That is, if I dragged c:\temp\dummy.jpg onto TW, I get the tiddler that includes
[img[dummy.jpg]]
But "dummy.jpg" no longer displays.
I have to fix the the long path like this:
file://c:/temp/dummy.jpg
below
!file
in the attachment tiddler. Then it works.
If I drag a text document onto the TW, a new tiddler is made and the text is inserted as before.
Mark