Neither Mozilla nor Thunderbird issue email addresses or passwords for mail accounts. You email address will most likely have the name of the company that issues it following the @ in your email address. They are the ones you need to contact if you have forgotten your passwords for the mail account. The password is usually the same one you use to access your mail from a browser on their web site.

The only emails about password resets you will get from Mozilla will be resetting the password for this site, or other Mozilla owned web sites.

My guess is your password stopped working because Thunderbird is trying to log into a mail account with a provider you no longer have an account with.

Have you added your new account to Thunderbird? On the toolbar > Click the > beside New message and select existing mail account from the menu.

You don't understand. I changed my email provider but did not change it in TB. Since TB won't let me in, I can't change it. And when I tell TB I forgot my password, it sends the reset email to the old email address which I can't access. It needs to send it to my new address

No you obviously do not understand.

When you are asked for your password and press cancel you will be in Thunderbird, just unable to collect mail from your no longer existing mail account.

I still don't know what you're telling me. TB is collecting mail from my new comcast account. The problem is getting on the TB forum, not into TB

But you're here on the forum. So what's the problem?

But what on earth does this mean?

And when I tell TB I forgot my password

I'm not on the forum. I had to go to Mozilla support for help. I can't get on the forum. It wants my password and since I don't know it I tell it I forgot it. But the mail from the forum goes to my old internet provider's email server. So, how can I get access to the forum to change my email address?

Yes you are on the support forum. To change settings try editing your profile. https://support.mozilla.org/en-US/user/johnG9210

I looked in your profile and the address associated with it is Comcast.

my mozilla.org profile is ok. What I can't do is get into the Thunderbird forum.

Er, this is the Mozilla support forum for Thunderbird. See at the top where it says "mozilla support" and "Home > Support Forum > Thunderbird".

If you're trying to get into another forum (which one?) we really can't help you with that.