These simple steps have worked for many. I hope for you as well. Here are the basics:

- change your security authorization to Oauth2 (do this for the incoming server settings and also for the Outgoing (SMTP) server - two places - remove the password entries for the account (remove the entire entries both incoming and outgoing) - that password info is at Tools>Preferences>Privacy&Security - click 'saved passwords' Then click 'show passwords' then highlight the rows with incoming and outgoing gmail servers and click DELete. - check to ensure that TB is set to accept cookies (that setting is just above the Passwords section) - Google will then step you through the authentication process for your PC.

I spent 4 hours doing all the things you suggested, several times, and rechecked them. None worked. I have also tried other things like disables of anit-virus and firewalls. None worked. I have found nothing that worked since Google changed things. Yes, I have checked everything I can think of in my Gmail accounts (3). Nothing I've changed or tried has worked. Gmail has not granted access to TB since June 2. I've even deleted TB and loaded a fresh app copy. Thanks for trying, but no solution found anywhere on the internet has worked.

Have you actually managed to get through the oAuth login process and get a key? This involves a number of web pages generated by Google.

The reason disabling anti virus is suggested is because some block web pages from not known browsers.

Do you have anything installed of listening on "LocalHost" I have heard of folk with web browsers (appache) usually) developer tools and some other software that has interfered by consuming the final authorization post on localhost? Using the netstat -b command from an admin command prompt will list everything listening on any port.

The authorization is a web page, served by Google. ANything that consumers the message on localhost with the authorization key will break the process.

Generally using the operating system in safe mode with networking will load a basic system without the products that might break the authentication process. So I suggest you try that, but a wired connection to the internet is required as WiFi is not available in safe mode with networking.

The disable of anti-virus did not work. All firewall settings were checked again also. Nothing worked.

The message remains the same: TB will not login to Gmail.

I also tried changing everything from imap to only pop. I get the same message: Pop@gmail.com responded won't authenticate password.

Then I tried changing to only imap. imap@gmail won't authenticate password.

I went back and tried both at the same time. Nothing worked.

I also set a new password in case something was wrong with the old password. Didn't change anything. TB won't log in. Yes, I deleted all old passwords in TB.

The last time I checked TB passwords there's a long, strange, password with many letters and numbers. It has a little globe icon beside it. I think it's the password that Gmail encrypted, and put in TB, after GM authorized TB.

A message from GM said TB was authorized to read and modify email. Went through 2 different pop up windows to authorize two accounts

The latest snag is TB has lost everything in one of my accounts, #2, but everything in #1 & #3 are unchanged. Nothing remains in TB for #2 except the account name and email address. All folders & messages have disappeared in #2. (totally blank, the only folder is inbox-totally empty).

Yes, account name, email addresses and all messages for #2 remain on Gmail. Noting was deleted.

It is only in TB where #2 disappeared. The #2 was for personal (family stuff), and #1 and #3 are for work. However, that usage does not explain #2's disappearance.

I'm looking to find where TB stores profile files. Maybe I can restore #2. TB may have done something to #2 when I downloaded a fresh copy of TB. About in TB says: 102.0 (32 bit) up to date.

When all these problems started, I also did a MozBackup and stored it on my external, D- backup drive. Maybe the messages for #2 are there and can be restored.

New Suggestions ?

Also dealing with the same issue and have seen all the same solutions that never work. This is a major problem for people who rely on this to access multiple accounts.

I have completely disabled all firewalls, antivirus, etc. I have done every single thing on every list on every forum question.

None of it works.

I really hope someone can figure this out asap. I've been a loyal fan of Thunderbird and donate consistently but this is bothersome.

@kemblue: do you have 2FA enabled on your gmail account ?