What exactly happens? Any error messages?

Nothing happens. My login has no effect.

Open the Error Console by pressing Ctrl+Shift+J on your keyboard. In the Error Console window, click the trash bin icon at the top-left to clear it of all messages. Switch to Thunderbird's main window and click the Get Messages button to fetch mail for the problematic account. Are there any errors logged in the Error Console?

Also, what are your current server settings, for both incoming and outgoing? Post a screenshot if you so wish.

NS_ERROR_NOT_IMPLEMENTED: Component returned failure code: 0x80004001 (NS_ERROR_NOT_IMPLEMENTED) [nsIRequest.name] 3 OAuth2.jsm:171 NS_ERROR_NOT_IMPLEMENTED: Component returned failure code: 0x80004001 (NS_ERROR_NOT_IMPLEMENTED) [nsIRequest.name] 2 OAuth2.jsm:171 NS_ERROR_NOT_IMPLEMENTED: Component returned failure code: 0x80004001 (NS_ERROR_NOT_IMPLEMENTED) [nsIRequest.name] OAuth2.jsm:171 Empty string passed to getElementById(). browser-custom-element.js:391:22 TypeError: PopupNotifications is undefined LoginManagerPrompter.jsm:776:24 NS_ERROR_XPC_JAVASCRIPT_ERROR_WITH_DETAILS: [JavaScript Error: "PopupNotifications is undefined" {file: "resource://gre/modules/LoginManagerPrompter.jsm" line: 776}]'[JavaScript Error: "PopupNotifications is undefined" {file: "resource://gre/modules/LoginManagerPrompter.jsm" line: 776}]' when calling method: [nsILoginManagerPrompter::promptToSavePassword] LoginManagerParent.jsm:1018 NS_ERROR_NOT_IMPLEMENTED: Component returned failure code: 0x80004001 (NS_ERROR_NOT_IMPLEMENTED) [nsIRequest.name] OAuth2.jsm:171 NS_ERROR_NOT_IMPLEMENTED: Component returned failure code: 0x80004001 (NS_ERROR_NOT_IMPLEMENTED) [nsIRequest.name] 2 OAuth2.jsm:171 Some cookies are misusing the recommended “SameSite“ attribute 2 NS_ERROR_NOT_IMPLEMENTED: Component returned failure code: 0x80004001 (NS_ERROR_NOT_IMPLEMENTED) [nsIRequest.name] 3 OAuth2.jsm:171 NS_ERROR_NOT_IMPLEMENTED: Component returned failure code: 0x80004001 (NS_ERROR_NOT_IMPLEMENTED) [nsIRequest.name] OAuth2.jsm:171 NS_ERROR_NOT_IMPLEMENTED: Component returned failure code: 0x80004001 (NS_ERROR_NOT_IMPLEMENTED) [nsIRequest.name] OAuth2.jsm:171 NS_ERROR_ABORT: User canceled master password entry 3 crypto-SDR.js:90 NS_ERROR_FAILURE: Couldn't decrypt string 2 crypto-SDR.js:200 NS_ERROR_NOT_IMPLEMENTED: Component returned failure code: 0x80004001 (NS_ERROR_NOT_IMPLEMENTED) [nsIRequest.name] 3 OAuth2.jsm:171 Content Security Policy: Ignoring “'unsafe-inline'” within script-src or style-src: nonce-source or hash-source specified Content Security Policy: Couldn’t process unknown directive ‘require-trusted-types-for’ Content Security Policy: Ignoring “'unsafe-inline'” within script-src or style-src: nonce-source or hash-source specified 2 NS_ERROR_NOT_IMPLEMENTED: Component returned failure code: 0x80004001 (NS_ERROR_NOT_IMPLEMENTED) [nsIRequest.name] OAuth2.jsm:171

GMAIL ACCOUNT SETTINGS

==========

Account Name Gen’l Delivery

E-mail Address WPeyser@gmail.com

“From” Name William Peyser

Incoming Server (POP3) pop.gmail.com

Incoming Port 995

Incoming Security SSL/TLS

Incoming Authentication Normal password

Incoming User Name WPeyser@gmail.com

Incoming Password ********

Outgoing Server (SMTP) smtp.gmail.com

Description Google Mail

Outgoing Port 465

Outgoing Security SSL/TLS

Outgoing Authentication Normal password

Outgoing User Name WPeyser@gmail.com

Outgoing Password ********

Security Question Library card number?

Answer 5589 or 12345

GMAIL ACCOUNT SETTINGS

==========

Account Name Gen’l Delivery

E-mail Address WPeyser@gmail.com

“From” Name William Peyser

Incoming Server (POP3) pop.gmail.com

Incoming Port 995

Incoming Security SSL/TLS

Incoming Authentication Normal password

Incoming User Name WPeyser@gmail.com

Incoming Password ********

Outgoing Server (SMTP) smtp.gmail.com

Description Google Mail

Outgoing Port 465

Outgoing Security SSL/TLS

Outgoing Authentication OAuth2

Outgoing User Name WPeyser@gmail.com

Outgoing Password ********

Security Question Library card number?

Answer 5589 or 12345

GMAIL ACCOUNT SETTINGS

==========

Account Name Gen’l Delivery

E-mail Address WPeyser@gmail.com

“From” Name William Peyser

Incoming Server (POP3) pop.gmail.com

Incoming Port 995

Incoming Security SSL/TLS

Incoming Authentication OAuth2

Incoming User Name WPeyser@gmail.com

Incoming Password ********

Outgoing Server (SMTP) smtp.gmail.com

Description Google Mail

Outgoing Port 465

Outgoing Security SSL/TLS

Outgoing Authentication OAith2

Outgoing User Name WPeyser@gmail.com

Outgoing Password ********

Security Question Library card number?

Answer 5589 or 12345

Whatever you do, do no not post your email address or other personal/sensitive information here.

You need to change Authentication Method to OAuth2 for both incoming and outgoing Gmail servers. This change should have happened automatically, but for some reason that is not the case on your end. See Automatic Conversion of Google mail accounts to OAuth 2.0 Authentication

I see an error stating NS_ERROR_ABORT: User canceled master password entry. Have you ever set a master password? If so, did you get a prompt for it then cancelled it?

Incoming and outgoing servers ARE set to OAuth2. No, I didn't set a master password.

Do you get a prompt to sign in to your Google account?

Yes, repeatedly, but the sign in has no effect. Same is true for my Comcast email account. I sign in with a user name and password, but the sign in has no effect. I can't send or receive messages in either account.

Go to Preferences > Saved Passwords. Delete all entries. Still under Preferences, go to Show Cookies and remove all of them. Restart Thunderbird.

Did that. Had no effect. Still can't send or receive messages.

Try a clean installation of Thunderbird. Go to C:\Program Files AND C:\Program Files (x86) and delete the Thunderbird folder. Go to thunderbird.net and download a fresh copy then install it. Launch it and see if the issue persists.