Surely comcast provide a webpage where you can work with your email? And very likely you can also change the password for yourself too.

Go to the Comcast webmail page and see if you can log in there. If you can then your username and password are fine. Incorrect server settings can also cause the password error. You might need to update those.

baseball6830 said

I'm trying to get into my Thunderbird e-mail account, using the password I always use, but it won't work. The system responds, "Sending of username did not succeed. Mail server mail.comcast.net responded: internal server error." I tried to get help changing my password, but the instructions are sent to my e-mail address, which I can't access! Thanks for any advice you might have.

Thanks very much. The problem has been resolved.