If all else fails, may have to go back to Outlook
No, don't do that! It works just fine with Thunderbird. It really does. You just have to make sure the setup is correct. The key thing is the settings in Thunderbird's server settings.
1) In T'bird, click Tools->Account Settings
2) Select the "Server Settings" item in your e-mail account
3) Ensure the server name is "127.0.0.1" on Port 110
4) Set the username as follows: mailbox-user-name#pop-server-name
For example, if your mailbox user is "myname@myisp.com" and the pop server is at "pop.myisp.com", you would enter "myname@myisp.com#pop.myisp.com"
Repeat 1-4 for any other e-mail accounts you are using in T'bird.
Finally, set up your Outgoing Server (SMTP) settings. Same dialog, at the bottom of the list:
1) set the server name as "127.0.0.1" and accept the default port number (usually 25)
2) check "Use name and password"
3) set up the user name. Again, it's of the form: "smtp-user-name#smtp-server-name".
How you do this depends on whether your ISP's SMTP server requires authentication or not (i.e. if you have to log on to their SMTP server).
If you do, then use your login name as "smtp-user-name" and the smtp server name as "smtp-server-name" above. For example, "mysmtpusername#smtp.myisp.com".
If you don't have to log into the SMTP server (which is the case with most ISPs if you're logged in with them already), then you can omit the "smtp-user-name" bit. In this example, the "user name" would be just "#smtp.myisp.com"
4) I'd try leaving "Use secure connection" set to "no" for the time being.
That really should do it. I agree, its a bit of a pain when Norton can do all of this totally seamlessly, regardless of which e-mail client you use. Still, once it's set up in Avast!, it does seem to work well.