reference https://support.mozilla.org/en-US/questions/1455439

I'm having so many problems with Thunderbird (after many years of no problems) that I think I need to reinstall Thunderbird. Do I need to uninstall Thunderbird prior to reinstalling it? I'm desperate for help.

I've attempted to change settings and I'm thinking that possibly I've screwed it all up.

What settings did you attempt to change and why?

I've changed the passwords through Comcast a few times

Why would you want to change passwords multiple times?

Can you login to your account via webmail?

I have constant problems signing in with the same old password so I've contacted Comcast to change the passwords.

It doesn't work that way. Comcast cannot change the password for you. Only you can set the password on the server.

Is there anywhere on Thunderbird where I can check to see what passwords are still listed for me?

All you can do is to tell Thunderbird to remember the password you set on the server, so that you don't get prompted for the password in Thunderbird again and again. You cannot change the account password via Thunderbird though.

To check which passwords Thunderbird has remembered: At the top right of the Thunderbird window, click the menu button ≡ > Settings > Privacy & Security > Saved Passwords > Show Passwords

You may want to delete old (and now obsolete) passwords Thunderbird has remembered.

Also, please post your Troubleshooting Information. At the top right of the Thunderbird window, click the menu button ≡, then select Help > More Troubleshooting Information. Press the "Copy text to clipboard button" and paste the information into your reply.