I can access my account directly from Firefox, but not through Thunderbird. NO changes had been made to my computer between the aforementioned times.

My Gmail POP and SMTP stopped working today as well. I updated to Thunderbird 91 and changed the following settings:

1. Select the `Account Settings` menu item from the `Tools` menu.

2. Click on your Gmail account and choose `Server Settings`.

3. Change the `Authentication method` pop-up menu to `OAuth2`.

4. Click on your `Outgoing Server`. Click the `Edit` button and change the `Authentication method` pop-up menu to `OAuth2`.

That solved the problem for me.

Thanks. I still can't connect.

When I try with those settings, I get the window below. First it has my email address in the window. Once I hit next the window goes blank with "Email or phone" ghosted, as shown.

If I click on "Find My Account" and fill in my recovery email, Google tells me I need to turn on Cookies. I never turned them off between my last connection and when I found Thunderbird no longer to be working. (times in first post.)

Apparently somebody ate all of my cookies and Google can't find them.

So... I went into my Thunderbird Tools > Preferences and played with the cookie settings. Google window popped up and I had to log in. Another google window popped up with "Allow" and "Deny" buttons. (Allow or deny Thunderbird access to my google email account.) Upon clicking on the Allow button, 30+ emails showed up, but... when I once again clicked on "Get Messages" I was right back to my original problem.

Looks like I just need to keep playing with my cookies until I toss them.