1. first, click account settings and 'synchronization&storage' and the advanced tab to verify that the desired folders may be downloaded
2. while on that pane, verify that you have sychronization locally regardless of age set.
3. now, click file>offline/download from menu bar (if menu bar not shown, click alt key first)
4. this may take time to get all headers and content locally.
5. you now have the messages local, but they're still married to the IMAP account. your next step is to separate that. several options '
   • if you feel comfortable working in windows file manager, then exit TB, locate the message folders in the Imapmail\account folder and copy them to the Mail\Local Folders folder. restart TB, verify that all is well, and then close the IMAP account. And do BACKUP.
   • OR, you can install the importexporttools addon, then highlight each IMAP folder, rightclick and select importexport option to 'export as mbox' - then, highlight Local Folders, rightclick and select importexport option to import mbox. repeat for each folder. check that all is there, then delete the IMAP account, and do BACKUP.

I appreciate the help. I don’t see anything about “synchronization” under Settings, neither TB nor Comcast settings.

I deleted the POP3 account on TB and am now trying to set up IMAP and will then try to continue with step 3. (Thank you for the Alt key tip to see the menu bar. I wondered where it was. A very basic problem without an obvious solution.)

For now, even though I am using the configuration for Comcast which is provided by TB, and listed in the Xfinity site, I am getting error messages that it is the wrong configuration and it con’t log into Xfinity server to connect. Who knows… It’s always something! I will keep trying and then follow the rest of your advice. Thank you.

The synchronization settings apply only to IMAP accounts and that pane appears along with other related panes, such as server settings.

I suspected that. Still no luck setting up the IMAP account. Same error messages -- configuration and/or password. Verified setting and password a dozen times, Finally reached Xfinity support and spent 45 minutes with them and they said my settings were correct and they could find nothing wrong with my comcast email account. And told me to phone Thunderbird 😊😊 ! I even shut down and restarted the computer. Tomorrow I will try to delete and re-install TB. Who knows… If it won’t work I will try Outlook or another client. It doesn't make sense but computers always have mysterious gremlins! Again, I thank you for your help and have learned more along the way.

This thread looks similar and may prove useful:

https://support.mozilla.org/en-US/questions/1437228

Thank you. I have been using those settings (as shown in the solution). And looked at the other suggested pages, which I have already seen. And have tried a few variations of the port settings and SSL settings just for the heck of it. Always the OAuth2. And have tried with and without the password, hoping it would give me another chance to enter it.

It takes me to the Xfinity login page, with my userid filled in. I click “Let’s go”, the usual way to open the page to enter the password, and the next page is “Access Denied, you don’t have permission to access http:/logon.xfinity.com/login on this server.” I have tried googling that message but nothing answers my problem.

I suspect it is an xfinity problem somewhere and will DM them again and hope I get a higher tier of help, though she said she had messaged the third tier when I tried yesterday.