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

Thanks Christ1, I pasted the information after pressing "Copy Text to clipboard button" and I got this message: 'Ensure this value has at most 10000 characters (it has 10126)' . The last part of the info shows a lot of info on 'Graphics'. Could I delete all of that part, 'Graphics', so as I can reply to you with it?

Could I delete all of that part, 'Graphics', so as I can reply to you with it?

Yes, you can delete that part.

OK, here is info with Graphics info removed.

 Application Basics

   Name: Thunderbird
   Version: 78.14.0
   Build ID: 20210901192859
   Distribution ID:

   Update Channel: release
   User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.14.0
   OS: Windows_NT 10.0

   Launcher Process: Enabled
   Multiprocess Windows: 0/0
             Disabled
   Remote Processes: 0
   Enterprise Policies: Inactive
   Google Location Service Key: Missing
   Google Safebrowsing Key: Missing
   Mozilla Location Service Key: Missing
   Safe Mode: false

 Mail and News Accounts
   account1:
     INCOMING: account1, , (none) Local Folders, plain, passwordCleartext

   account36:
     INCOMING: account36, , (imap) imap.mail.yahoo.com:993, SSL, OAuth2
     OUTGOING: , smtp.mail.yahoo.com:465, SSL, OAuth2, true

   account37:
     INCOMING: account37, , (imap) imap.mail.yahoo.com:993, SSL, OAuth2
     OUTGOING: , smtp.mail.yahoo.com:465, SSL, OAuth2, true

   account38:
     INCOMING: account38, , (imap) imap.mail.yahoo.com:993, SSL, OAuth2
     OUTGOING: , smtp.mail.yahoo.com:465, SSL, OAuth2, true

   account39:
     INCOMING: account39, , (imap) imap.mail.yahoo.com:993, SSL, OAuth2
     OUTGOING: , smtp.mail.yahoo.com:465, SSL, OAuth2, true

   account40:
     INCOMING: account40, , (imap) imap.mail.yahoo.com:993, SSL, OAuth2
     OUTGOING: , smtp.mail.yahoo.com:465, SSL, OAuth2, true

   account41:
     INCOMING: account41, , (imap) imap.mail.yahoo.com:993, SSL, OAuth2
     OUTGOING: , smtp.mail.yahoo.com:465, SSL, OAuth2, true

   account42:
     INCOMING: account42, , (imap) imap.mail.yahoo.com:993, SSL, OAuth2
     OUTGOING: , smtp.mail.yahoo.com:465, SSL, OAuth2, true

   account43:
     INCOMING: account43, , (imap) imap.mail.yahoo.com:993, SSL, OAuth2
     OUTGOING: , smtp.mail.yahoo.com:465, SSL, OAuth2, true

   account44:
     INCOMING: account44, , (imap) imap.mail.yahoo.com:993, SSL, OAuth2
     OUTGOING: , smtp.mail.yahoo.com:465, SSL, OAuth2, true

 Crash Reports for the Last 3 Days

 Remote Processes

   Type: Count

 Extensions

     Name
     Version
     Enabled
     ID

       Amazon.com
       1.1
       true
       amazondotcom@search.mozilla.org

       Bing
       1.0
       true
       bing@search.mozilla.org

       DuckDuckGo
       1.0
       true
       ddg@search.mozilla.org

       Google
       1.0
       true
       google@search.mozilla.org

       Wikipedia (en)
       1.0
       true
       wikipedia@search.mozilla.org

 Security Software

   Type: Name

     Antivirus: Microsoft Defender Antivirus
     Antispyware:
     Firewall: Windows Firewall

 Graphics

     Features

Your server settings look good.

Can you try to start Windows 10 in safe mode with networking enabled?

Windows safe mode disables anti-virus software. So this isn't a solution, but a troubleshooting step to determine whether anti-virus software is the culprit.

I'll close out of everything & restart as suggested. Then report back to you.

Well Christ1, in safe mode w/ networking enabled, Thunderbird loaded & I got Thunderbird pop-up window: "Failed to connect to server imap.mail.yahoo.com." I checked Firefox & go to 'Homepage' but no bookmarks would load either: "We can't connect to server at..." What does this mean as far as determining whether anti-virus is the culprit?

That looks like a networking problem. Can you confirm you did start Windows in safe mode with networking enabled? If you use a WLAN connection this may not work in safe mode. So a wired connection would be preferred.

Hello again Christ1, I had to locate an Ethernet cable to set up wired connection. I did as you asked and I get the same result in Safe Mode. After a few minutes and clicking on different file folder emails, the same Thunderbird window pops up: [ "Server imap.mail.yahoo.com. has disconnected." ]

I was wrong before though, I immediately opened another tab in Firefox and all my bookmarks do work. There is no interruption in connection online. Just Thunderbird.

The other thing I checked: Thunderbird disconnects for a few minutes at a time & then starts working again. I can work with it for a while and then [ "Server imap.mail.yahoo.com. has disconnected." ] pops up, so I work on stuff for a few minutes and re-click on a folder and it's working again for a while. It just won't stay connected continuously is the problem.

Hope this info helps you, please advise. Thanks much.

So then we can remove anti-virus software from the list of potential culprits.

Are you accessing the server from multiple devices simultaneously? You can try to reduce the "Maximum number of connections to cache" for any of the affected Yahoo account(s). The option is in Account Settings - Server Settings - Advanced. Mine is set to a value of 3, which works for Yahoo.

OK, I checked and the settings were set at a value of '5'. I changed it to a value of '3'. I'll see how that does today & tomorrow as I work the e~mail folders. Thanks, I'll let you know then.

Christ1, Wow, I'm so relieved & happy that it worked, reducing the "Maximum number of connections to cache" to a value of '3' from '5'. My Thunderbird account with Yahoo accounts has worked flawlessly ever since. Thank you for restoring confidence with Thunderbird. It's guys like you who make our little stupid private world out here better. For that I'm making another donation to MZLA Technologies. Again "Thanks" so much to your patience and competence. GoCaT2

Christ1's solution solution works for me also. Glad to find this post.

This may be a common problem and may be a good case to be added in Thunderbird and Yahoo help page or even as default in the wizard.

The connectivity issue is not specific to Yahoo hence why it is not under the Thunderbird and Yahoo' page, but it is menttioned in this article under the 'Advanced IMAP settings' https://support.mozilla.org/en-US/kb/configuration-options-accounts#w_advanced-imap-settings

re :For that I'm making another donation to MZLA Technologies.

That would be appreciated. Here is a link to correct donation forum. https://give.thunderbird.net/