Since you are asking for help in the Thunderbird forum I will focus on your request for help with Thunderbird.

Having an issue with Thunderbird/comcast email account in that messages are apparently sent, but don't display.

What do you mean by "but don't display"? The messages don't show up in your Sent folder in Thunderbird?

Please be specific.

Asked that help be sent to Firefox/Yahoo account so could read. Firefox asked to change it's password. Did so. New password does not work. Changed three times and still doesn't work. Old pw doesn't work either.

It sounds like you access your Yahoo e-mail through webmail, using Firefox as your web browser. Yahoo told you to change your Yahoo password, so you did. Then there were continuing problems.

These problems are with Yahoo, so go here to get Yahoo support: https://ca.help.yahoo.com/kb/mail-for-desktop

Had some issues with setting up "Forum account". It didn't like the Firefox/Yahoo account so switched to Firefox/Microsoft account.

When setting up the forum account here, you need to verify it. The account is verified by sending you an e-mail to the e-mail address you gave. In that e-mail is a link to click on to confirm the forum account. Since you gave your Yahoo e-mail address, and you currently cannot access your Yahoo e-mail, this is obviously a problem. But you solved it by using a different e-mail address.

By "didn't display" I mean that upon opening my Mozilla/Thunderbird account, new emails are first downloaded. [Can watch at bottom of screen.] Upon completion of the download, the subject of the email is presented near the top of the screen and clicking on the subject opens the email. What happens is the download is completed, but then nothing happens. I.e. the subject does not appear. I sent an experimental message to my account from another source and it appeared to be in the download operation, but also did not subsequently "display" it's subject. I have had this happen before, but closing the account and reopening it [rebooting?] would make it work.

The problem has been solved!! After posting the reply above, I opened the problem Thunderbird/Comcast account for about the 4th time and found that an update was available. Not available before. Installed the update and ----- everything worked! Thank all of you for your support !!!