given the password used by gmail for IMAP and SMTP are the same as the one you use when you log into their web site. The password being asked for should be simple. But it is not.

Nortons and some other anti virus programs store the passwords and when they are changed generally make a complete mess of things. Continuing to quietly substitute the new correct password you type for the old incorrect one they hold. Two factor authentication only works with mail application if you create an application password on gmail for use by Thunderbird. These can be invalidated by gmail for a number of reasons that are somewhat unclear and have to be recreated.

I think you are saying I'm screwed! I do use Norton and LastPass but how do I get past the SMTP/IMAP login problem? I'm not using 2factor auth. Thanx

disable the password vault in Norton, and email scanning. It can not scan mail on encrypted connection and non encrypted connections are getting less and less. So you might as well save the overhead and the hassle of having it thing it is doing something. You also will be better of without the false sense of security you have not about your email.

Last pass, I really do not know how it works, but I would assume you can locate and remove passwords in the program.

Tried suggestion, no effect. Could I resolve prob if I deleted Thunderbird and reinstalled thereby starting with a clean slate? I have a MozBackup of everything. Thanks for help.

You have a configuration problem. Configuration is managed by your profile. The program itself is installed separately. Uninstalling and reinstalling the program won't affect the configuration which is stored in your profile.

As Matt has indicated, Gmail can be tricky as it offers several ways to authenticate. This gives it flexibility to work with multiple email clients of varying degrees of sophistication. But this leaves us with having to choose one method amongst all the options offered.

My several Gmail accounts all now use OAuth2. Thunderbird knows about this, so at the appropriate point, a web browser screen appears inviting you to enter your gmail account's password. This is a lot less hassle than having to go to the Gmail site to generate an application-specific password.

So in addition to trying to stop Norton (et al) from interfering, I'd go to Tools|Options|Security|Passwords->Saved Passwords, remove all the entries associated with this account, and then restart Thunderbird.

I use Last Pass myself but it is almost entirely irrelevant to the use of an email client. It may be able to help out when that Google login prompt appears, but as it's a browser plugin or extension, I'm not sure it can "see" what's happening in an email client.

I've removed passwords from Tools|Options|Security|Passwords->Saved Passwords, to no avail when restarting. Could I review the OAuth2 file to obtain/change the IMAP and SMTP pw to so I can enter valid ones when asked by Thunderbird? How to access OAuth2 file? Thanks!

There is no "OAuth2 file" that I know of. All that Thunderbird needs to know to connect to a Gmail account appears to be stored in Tools...Saved Passwords.

That is my problem. I don't know the passwords for imap.google.mail.com server or smtp.googlemail.com. I need to reset them to a known value? Can they be accessed? Can the Profile be changed or deleted? Thanks.