You may want to check your allowance of 3rd party apps in your Google Security. Recent changes in Google made me have to go back in and re-enable Thunderbird there.

Go to "myaccount.google.com" Click on the SECURITY Link. Look for "Your connections to third-party apps & services".

It should show "Mozilla Thunderbird Email"

If it doesn't, you need to add it.

I'm on my 'Google Account - Security - Your devices' page now. And apart from the 3 top R/H regular icons and the 4 Info links at the bottom, it lists '2 sessions on Windows computer(s) including today and 1 session (old) on Android phone (along with the forward '>' to history. Nothing else and no other visible links to other actions, so am I in the right place??

This is my Gmail Server setup at the moment . . .

I'm on my 'Google Account - Security - Your devices' page now. And apart from the 3 top R/H regular icons and the 4 Info links at the bottom, it lists '2 sessions on Windows computer(s) including today and 1 session (old) on Android phone (along with the forward '>' to history. Nothing else and no other visible links to other actions, so am I in the right place??

Yes but you need to look below 'Your Devices' to see : 'Your connections to third-party apps and services' Thunderbird should be in the list. If it is not in the list then that explains why gmail need you to enter password.

Please make sure you do not use 2 step verification in gmail because that is entirely different and you cannot use oauth. Check - access webmail account and make sure 2 step verification is switched off. It's in the google account > security section same place as mentioned above.

It is likely there is nothing wrong with the server settings in Thunderbird and outgoing smtp is perfectly ok.

You set everything up to use OAuth, so gmail will now only accept an oauth token. You cannot use an oauth token when entering a manual password. However, if a password gets reset in the webmail account then the oauth token is no good as it needs resetting as well. When you enter a password it's a normal password same one you use to access webmail but gmail says no because it expects an oauth. Now you're stuck in a loop.

If you have a program on your computer that is able to clean up things like session and password files eg: CCleaner, WiseCleaner then please make sure that program has your thunderbird profile folder as exempt from access/scanning. Note some anti-virus products can also do this. If these products mess about with thunderbird password files then it causes havoc.

In Thunderbird Access passwords and remove them

• Settings > Privacy & Security
• Passwords section
• Click on 'Saved Passwords'
• click on 'Show Passwords'

You may see any of the following lines for that gmail account:

• imap://account..
• mailbox://account....
• smtp://account....
• oauth://account.....

You need to select the line that mentions the gmail account and click on Remove. Make sure all lines refering to your gmail account have been removed.

then restart Thunderbird and gmail will auto prompt for password. It must be the identical password you use when you want to logon to the gmail email address webmail account.