For starters the D&D events are being masked out ni nsDocumentViewer.cpp then the next issue is that the event are going to the PageFrame which doesn't have any content so they get thrown away.
Changing component from printing to print preview.
This could be addressed by bug 133787.
what would one want to D&D into a print preview window?
> This could be addressed by bug 133787. this bug may be complicated than 133787, but it can at least be a blocker.
shouldn't this be severity=enhancement?