This is a known issue in Firefox when using the Editor in a Window. The browser reloads the Editor iframe so the HTML is cleared and the content becomes not editable. We are currently investigating what is causing this behavior and will do our best to fix it for the next releases. A workaround for now is to trigger the "DOMNodeInserted" event on the container in the edit event:
This will cause the editor content to be recreated and it should work as expected.
the Telerik team
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