
You can't use the extensions in IE mode because IE mode doesn't support extensions that interact with the IE page content directly. You can refer to the last point in this paragraph IE mode supports the following Internet Explorer functionality.
I think there's no other extensions we can use because if we want to take the full page screenshot, the extension must interact with the IE page content which is not supported by IE mode. In IE mode, I think you can only take a screenshot of the full screen like using PrintScreen in the keyboard.
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Regards,
Yu Zhou