[oh8eleven]

I love Thunderbird, but if you ever change your ISP, forget it. There goes your address.
Not sure what you mean. You can change your email host from Gmail, Hotmail, Yahoo, etc., in TB as often as you want.
And even if you have more than one email account on different servers, you can add them all to Thunderbird.
All you have to do is create a new account for each one within Thunderbird that points to the new server(s).
Google "how to set up thunderbird for XXXXX" where XXXX is your email provider, and you'll find detailed set up instructions.
Setup a new TB account for each email address.

[Windflier]

Not sure what you mean. You can change your email host from Gmail, Hotmail, Yahoo, etc., in TB as often as you want. And even if you have more than one email account on different servers, you can add them all to Thunderbird.

No, that's not what I'm talking about. It's when I've had to move my physical household, and have wound up with a new ISP where I've had problems.

In the past (admittedly a good number of years now), I've had to create a new email address after switching service providers. I wasn't able to keep my old email address when using Thunderbird.

With services like Yahoo mail, your account travels with you, regardless of who your service provider is. Are you saying that you can create a portable Thunderbird address now?