Firefox's toolbar customization interface is currently based on the very nice direct manipulation model used for OS X applications (the user can directly pick up controls and placing them where they want). However it currently has a number of limitations, including:
1) We need to be able to have multiple instances of the same control in the customization palette, like separate back and forward navigational controls and the combined keyhole navigational control (currently we can only have a single instance of each type of control).
2) The cursor should indicate when a control can not be dropped
things that can't be moved shouldn't be able to be picked up
3) Controls dropped in an area where they can not be dropped should animate back into place on the customization palette
4) On windows the cursor should not have a small rectangle under it if an object is being dragged (since the object itself is being drawn under the cursor)
5) Instead of drawing placement lines on the toolbar, controls should animate away making room for the new control to be dropped.
6) The visual appearance of the control being held should change based on where the user is intending to drop it. For instance, a control might have a normal styling if placed on the toolbar, but if placed inside of the location bar it might appear as an end cap button, on the title bar it isn't as tall, etc.
7) Toolbars need gripy notches on their left side and be drag-able to different positions
8) Every surface should be capable of holding custom controls, including the title bar and tab strip