How can I prevent the user from being able to resize an image in designMode? (disable the handles when image is clicked)
Although the question was a long time ago. Every block element (div, img...) will be decorated with handles from FF. Test it:
Sample text!
No handles, no resize etc.
Sample text!
The image can be resized. So you have to explicitly call contenteditable="false" for every block elem you do not want to have handles, as nickf said already for the images.
Even more weird: assign "position:absolute" to any of your elements - even the parent div - and it has handles again.