UR uses the standard
MAPISendMail method for File | Send (similar to what is used for right-click -> Send To -> Mail Recipient in Explorer). In my tests with Outlook 2007, Windows Mail, and Thunderbird 2.0.0.22 on Vista, making any of those the default mail client and restarting Windows causes that mail app to be used for File | Send in UR and Send To -> Mail Recipient in Explorer. I don't know how to diagnose if this isn't working for you. Maybe try uninstalling/reinstalling Thunderbird.