The default mail client is the program that opens when you click a link in another application to send a mail message.

If you have Thunderbird installed and you wish to make it your default email client then please follow the below:-

1. Open Thunderbird
2. Go to "Tools -> Options -> General"
3. Select "Use Thunderbird as the default mail application"

If that doesn't take effect right away, try restarting Thunderbird, or even restarting the computer itself.