RadEditor uses the browser's editable iframe for its content area and therefore it inherits the behavior of the iframe for the particular browser. Note also that the control does not support browsers add-ons, as there may be numerous scenarios for handling.
What I can suggest you, however, is to create a custom content filter that handles the particular add-on's behavior. Editor - Custom Content Filters
online demo as well as Content Filters
help article shed more light on the matter.
Should you have any suggestions on how the functionality of the RadEditor can be improved, do not hesitate to share them with us.
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