If you have .eml and .nws added to 'Tools | Options | Browser | File extensions to display in internal browser view,' and Windows is configured to open those file extensions in Thunderbird, IE/Windows (hosted in UR for the internal browser view) will show the message in Thunderbird. That's not something we have control over -- it's configured in Windows.
If you remove .eml and .nws from the above option and add it to 'Tools | Options | Documents | File extensions to display in MIME viewer,' UR will show the message in its internal MIME viewer (text format messages only).
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