Do you have a web server such as Apache listening on localhost? Are you allowing cookies in Thunderbird?

Not using a web server as far as I know. Cookies are allowed in Thunderbird. Thanks for looking into this.

I suggest you try with your computer in safe mode with networking and see if you get further. It is probably some dumb security product blocking or quietly scanning and failing in the background. Safe mode with networking does not load most of windows startup programs and often removes roadblocks.

I tried safe mode with networking and it still acts the same. I have been able to set up my gmail accounts in MS Outlook, so this seems to be a Thunderbird problem. I have another problem which may or may not be related: I have not been able to use Firefox for a while. I have an Acer computer running Windows 10. Acer had a trial version of MS Office which I did not subscribe to. I uninstalled this from my computer and immediately after I did this, Firefox stopped working. When I start Firefox, it comes up with blank pages and will not load any websites. I have never figured out why this should have happened and have had to switch to MS Edge for my web browser. (Firefox does not work in safe mode either.) The Firefox problem started quite awhile ago. The Thunderbird problem started only after Google started requiring Oauth2. I would appreciate any other suggestions. Thanks.

I made a mistake when I referred to MS Outlook. It is the MS Mail app that I am using for gmail now.

It's definitely not a Thunderbird problem because you're also having (long standing) problems with Firefox. Something in your environment is interfering with Firefox and Thunderbird network communications. Do you have another computer on the same internet connection that you could test Firefox and Thunderbird on?

I haven't used it for a long time, but yes I do. I tried it out and Firefox is working properly on it. Thunderbird does allow me to set up a gmail account with Oauth2 properly. So where do I go with my regular computer now? The blank pop-up window that I get appears to be the proper one to get the gmail credentials. It does seem like something must be blocking google.

So where do I go with my regular computer now?...It does seem like something must be blocking google.

Something is definitely blocking this. The usual culprit is security programs (antivirus and firewall), but since safe mode did not alter things, it's hard to say what else could be causing it. Also, this ventures well beyond the scope of help of this forum. All I can do is point you to other places where you might get a solution.

Try resetting your network settings as per https://www.majorgeeks.com/content/page/how_to_reset_network_settings_in_windows_10.html

You could also reset your firewall back to default settings as per https://www.thewindowsclub.com/reset-windows-firewall-settings

Confirm your hosts file hasn't been altered. If it has, restore it back to default https://winaero.com/reset-hosts-file-back-to-defaults-in-windows/

If none of this works, you may have to repair your Windows installation.

Tried resetting network settings, turned off zonealarm firewall, and restored hosts file. None of this worked. I am not sure if I want to go through with repairing the windows installation if it means reloading all my apps and losing all the app setting and histories, but I may try it when I have time to do it. If anyone knows anything else I can try, please let me know. Thanks to those that responded to my problem.

Would updating from Windows 10 to Windows 11 possibly fix this?

Went to update Windows and found an optional update for "Acer firmware". I installed this and it broke the computer. I could not get windows to boot up and got the blue screen with an "Inaccessible Boot Device" error. None of the repair mechanisms would work, so I was forced to reset windows to repair the installation. This worked and now my Firefox and gmail in Thunderbird are working. Now I have to go and reinstall and configure all the applications that were removed in the reset. This is going to take alot of time, but at least my computer is working the way it is supposed to.