I've suffered with this for weeks and finally worked around it. Thunderbird was set as the default client, but it didn't work with some other clients as well, so I went to Default Programs / Set Default Programs, selected Thunderbird, which was already set as the default mail client, selected 'Choose defaults for this program' and hit Save. That fixed it for PDF Complete, but not for FF4.

In FF4, I changed the mailto option to Always Ask (it had been Thunderbird(default)), then chose Thunderbird as the client after picking Send Link. that worked, so the next time I checked the box in the dialog to always use Thunderbird and now it works. grrr.

As I mentioned, in the content type window I don't have any options for mail or mailto. As you seem to have it it is not a FF4 thing. I wiped FF4 from my system and did a re-boot to be on the safe side. I re-loaded the latest FF4.0.1 and the Mailto setting has re-appeared - Hurah!!!!!! Now following the above instructions my mailto links work. Many thanks for taking the time to post :-D Dave