In Windows set Thunderbird as default email client.
You can change this from the registry, but careful when you edit the registry so you don't mess up things with the system
Start "regedit", and go to HKEY_LOCAL_MACHINE\SOFTWARE\Clients\Mail double click the (default) value and make sure it displays the correct client (in your case Mozilla Thunderbird). That did the trick for me!
I've been looking for this answer for a while myself. Thanks!!!
This worked for me as well. It is strange that if Thunderbird asks to be the default e-mail client, and you say yes, it will be used by windows itself (in explorer: send to > mail recipient), and MS Office (from menu, Send > E-mail), but not by Adobe Reader. This registry entry seems to be the problem.