One of the ‘new’ features in Firefox 3 is a graphical preview of the images, text, tabs, etc. your are dragging. This was suppose to make it easier for you to see where you were dragging the item. For text it is not too bad, but for images I found the preview makes it more difficult to properly drag and drop. Good news is you can easily turn off this feature and return to the familiar ‘box’ outline for drag and drop as in Firefox 2.  To turn off this feature:

1. In a new tab type about:config
2. In the filter field type nglayout.enable_drag_images
3. Double-click on the entry below so the value is false
4. Close the tab (no restart needed)