I'm not having that problem, but here are my settings. How do they compare with yours?

IMAP: Server name: imap.comcast.net Port: 993 user name: my Comcast email address connection security: SSL/TLS authentication method: Normal password

SMTP: Server Name: smtp.comcast.net Port: 465 User Name: my Comcast email address Connection Security: SSL/TLS Authentication method: Normal password

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You could try deleting your passwords for those servers from the password manager (Settings | Privacy & Security | Saved Passwords,) after which you should be prompted to input it again. I recently discovered that there were some leftover entries there that referenced old credentials - they weren't causing a problem for me, but maybe they are for you? I deleted them anyway.

I recently had a problem with another email account, not Comcast, where my mail stopped working in Thunderbird. I eventually deleted the account and re-added it, letting Thunderbird choose the settings from its database. That revealed that the provider had changed their settings without letting me know. It works now. I would not want to do that with Comcast because I have years and years worth of mail that it would take forever to download again, but if all else failed, I might resort to that.

Hi Lin. When did you start using imap with Comcast? When I changed to that I lost all my Comcast inbox messages and folders on Thunderbird. I changed back to POP and used my backup to recover the prior TB profile and got everything back. Thankfully.

My Comcast server settings delete email after 90 days.

I started using IMAP at Comcast many years ago. I don't even remember specifically using POP3 with Comcast, but I probably did in the earliest days. Their IMAP has always been very reliable - very few problems for me and I've never lost anything that I can blame on them - and it works best for the way I use email.

Rather than change the setting of your existing Comcast POP account to IMAP, it's safer to create an entirely new IMAP account in Thunderbird. Both can exist simultaneously. You can recreate your current folder structures in the new IMAP account or as local folders and copy messages into them. It's not likely that anything would go wrong, but a good safety measure would be to make a backup of your entire profile before proceeding.

They even have a support article on the process:

https://www.xfinity.com/support/articles/switch-email-pop-to-imap?

Hi Lin. When did you start using imap with Comcast? When I changed to that I lost all my Comcast inbox messages and folders on Thunderbird. I changed back to POP and used my backup to recover the prior TB profile and got everything back. Thankfully. My Comcast server settings delete email after 90 days.

I typed up a more detailed reply, but this board has, for some reason, decided to "hold it for review", which in my experience means it has disappeared for good. But the gist of it was:

...Rather than changed the settings of your existing POP3 account in Thunderbird, create a fresh, separate IMAP account. They can both coexist simultaneously. After you are comfortable that the new account works properly, you can delete the old one.

There are instructions in the support area of Comcast/Xfinity, but I don't dare to post the link here.