How do I Configure Thunderbird as the Default Email Client
Setting Thunderbird as your default email client lets other programs know to launch Thunderbird when you need to send an email. For example, when you surf the web with your web browser and click on a link to email a contact, your computer will start Thunderbird so you can send the email.
Thunderbird will ask if you would like to make Thunderbird your default email client during the setup process.
You can also set up Thunderbird as your default email client by going to the Tools menu and selecting Options. On the General tab in the Advanced panel, mark the check box next to Always check to see if Thunderbird is the default email client on startup.
You can also go to the Windows Menu and select 'Default Programs' in the right column above the 'Help and Support' link. Select 'Set your default programs'. Look for Thunderbird in the Programs listing and select 'Set this program as default'.