Has Norton come up with a fix for Thunderbird unable to send. Also, google searches with Firefox are blocked.
What email provider do you use? It is often not a Thunderbird problem, or a Norton VPN problem, but a problem with your email provider's protection for your accounts. I sometimes get email blocks from Gmail because they think someone from another part of the world is accessing my email account. So they block it until I can verify it is me that logged in from that virtual location.
I have found no way to change the security settings in Gmail to lower the protection levels to allow the VPN use. What generally has worked for me is to manually set Norton VPN's virtual location to be in my actual country.
This problem can be solved by turning off Norton VPN but that seems to be just a work around.