To find out how to setup Thunderbird as your mailto: default email client please click on the link below:

https://support.mozillamessaging.com/en-US/kb/make-thunderbird-default-mail-client

Please note: Depending on which browser you use you may also need to set up Thunderbird as the default email client inside your browser settings. 

If using Firefox please click here.

A link to Thunderbird support can be found here.