I opened Firefox on a freshly rebooted PC, went to click on a mailto: link in a webpage and get the error message that Thunderbird is already running. Thunderbird is not running, but will not open for sending email. The cause could have something to do with Firefox not opening Thunderbird correctly, or that Thunderbird is not getting the correct permissions to run properly when called by Firefox. When I start Thunderbird from it's icon, I get a Windows 7 prompt asking me if I want to allow this program to make changes on my computer. If Thunderbird is started by Firefox asking it to create a mail message, as is the case with a mailto: link, It never gets this permission requester. I have tried troubleshooting information regarding Thunderbird already running error messages, however, there are no .lock files associated with Thunderbird nor is it's ability to send/receive mail when run from it's own icon affected by this problem. I therefore think it's a problem with the way in which Firefox calls Thunderbird to create an email message. There is the additional problem that Thunderbird has to have administrative privileges to run, but doesn't automatically get them. This might require a separate bug report submission be generated to the Thunderbird developers to fix this problem, but is dependent on what they can do about that.