Hi Jelle,
The resize handles in Firefox and IE are generated by the browser's rich text editing engine and we cannot control that. You can read some details and a discussion on the matter here: http://www.telerik.com/forums/disable-drag-and-drop-of-body-element-in-content-area-iframe.
There is a way to disable them for images by setting attributes to them, but I cannot guarantee it will work in all browsers and cases: http://www.telerik.com/forums/how-to-solve-image-resize-anchor-that-is-visible-in-other-elements.
By popular demand, we have implemented a similar feature for Chrome and this is where you can disable it by adding the following function override at the end of the form that has the editors:
Regards,
Marin Bratanov
Telerik