It's always a security/password problem

Specifics about said problem would help. What exactly is the issue with passwords? Setting up Gmail in Thunderbird is pretty simple and straightforward. Thunderbird has inbuilt automatic server settings for Gmail and it defaults to secure IMAP and OAuth2 authentication. It's pretty much setup and forget, with very little customisation needed afterwards. You don't even have to tinker with Gmail or Google account settings to accommodate Thunderbird. It has been that way for at least five years now, so which version of Thunderbird are you using?

See https://support.mozilla.org/en-US/kb/thunderbird-and-gmail for details

My problem is similar but much more recent. I just tried updating this week from v68.6.0 to v78.1.1 (both 64-bit). All of a sudden I can't authenticate to Gmail. The errors are things like "failed to find the settings for your email account" and "Unable to log in at server". I'm setting it as IMAP, port 993, SSL/TLS, OAuth2 (outgoing is SMTP, port 465, SSL/TLS, OAuth2). I tried changing the setting within Google to allow less secure apps but no joy. I finally gave up, uninstalled v78, rebooted, reinstalled v68, and it works fine now. I also have Tbird accessing a Comcast email account, another for my church at a different host, and even an account from a community college where I teach. None of them will work.

So, what is it within the new Tbird that won't authenticate with Google or these other providers? At this point I'm gonna have to stick with the older version even if you tell me it needs bug fixes. I really, REALLY hate to have to access all of these different accounts via multiple browser windows.

Using 78.1.1 here, with 12 Gmail accounts in two separate profiles of 6 accounts each and they are all working just fine, with the automatically detected IMAP+OAuth settings. All accounts have 2-factor authentication enabled and none of them has "less secure app access" enabled (this is so not needed for Tbird anymore, for at least five years now). The OAuth2 webpage opens each time I'm setting up any of the accounts in a test profile. Not a single issue with passwords or reaching the servers. I have other non-Gmail accounts configured, also working as expected. I truly don't know what could be causing the problem you're having other than something to do with your network setup (firewall, vpn, antivirus, dns, proxy etc), so, what's your network setup like? Have you tried a new Thunderbird profile?

Thanks for your answers. I've toggled some settings and have been able to get it working again. I don't know for how long, and I don't how I got it working. I guess I'll have to pay close attention to it and see if I can figure out why it fails if it dies again.

I think there are at least two issues here.

1. You are trying to fix things by adding the account again. Do not do that it eventually causes it's own issues as you get a new numbered folder in your profile every time you do it and the old account data remains on your hard-disk. It is better to fix the setting for what you have as the auto detect in the config wizard is most unlikely to fix connectivity issues.

2. "failed to find the settings for your email account" is almost always a firewall issue as Thunderbird is reaching out to it's online database in the case of Gmail. If it can not get the settings it indicated that the anti virus/ Firewall is being ever helpful. Norton's product in particularly prone to blocking every new version of Thunderbird at their firewall for weeks after it is released. But it is not exclusively a Symantec product failure. They all do it from time to time.