"TinyMCE remove-base-64-image" immediately removes images dropped into the TinyMCE editor with Firefox and alerts the user with the reason.
I am aware that using setInterval to detect a freshly dropped image is not ideal from a theoretical standpoint as most of the time it's unnecessary and it could impact TinyMCE performance, albeit by a tiny amount. If you check the TinyMCE forums you will find another suggested method: Disable drag/drop facility (last post by Arvind, 2012-07-09) which involves modifying tiny_mce.js to detect the DOMNodeInserted event then deleting the new image node. While this seems sound, if you try to locate the '_addEvents function section' mentioned you'll find it's not in tiny_mce.js version 3.5.8-wp. I also searched the TinyMCE documentation for a way to bind this event in a plugin, but no joy there either.