Do you ever use Outlook? My experience is that on the rare occasion when I open it (usually to view a saved msg file) despite the settings set to prevent this, it always asserts itself as the default email client. I have Thunderbird set to check, when starting, if it is the default email client and to ask me if it isn't. This always gives me a prompt after having run Outlook. You could avoid all this by explicitly opening Thunderbird, creating an email message and adding the Word document to it as an attachment. You can drag and drop any file to send it as an attachment; drop it onto the addressing box in Thunderbird's compose window.