I think seeing your screenshots might be helpful to anyone who tries to help with your problem.

Adding Gmail Acct to Tbird - Update with image 2023-06-11

Having had another try, I now get the response "Configuration found in Mozella ISP database" (see pix).But no new Gmail folder(s) are yet to be found in Tbird. But maybe I'm a bit closer?

Something else occurs to me.  My (rather limited) little Doro "flip-fone for seniors" does not (yet) have email set up on it. Using it effectively for social contacting would not be an easy option for me - despite my having had to provide a phone number and also (in this event Waitrose email as) a Recovery Address to activate Gmail in the first place.  But must I include my phone in the system to make it all work?

Adding Gmail Acct to Tbird - Update with image 2023-06-11

Having had another try, I now get the response "Configuration found in Mozella ISP database" (see pix).But no new Gmail folder(s) are yet to be found in Tbird. But maybe I'm a bit closer?

Something else occurs to me.  My (rather limited) little Doro "flip-fone for seniors" does not (yet) have email set up on it. Using it effectively for social contacting would not be an easy option for me - despite my having had to provide a phone number and also (in this event Waitrose email as) a Recovery Address to activate Gmail in the first place.  But must I include my phone in the system to make it all work?

When setting up a gmail account, do NOT enter password. Google will handle that.

Misunderstanding maybe? My previously established Gmail works normally and I have prepared Gmail for the task with: Open - Settings - See all settings - Forwarding and POP/IMAP - Enable IMAP - Save changes.

But I get no further with Tbird than the point recorded in the attachment . There is a box requiring my (Waitrose I assume ?) Password, but I don't yet understand the resulting response.

I get to this point with: Tbird - Settings - Account Settings - Account Actions - Add Mail Account - Set up your Existing Email Address - 'Full name / Email address / Password (Tick: Remember PW)'

Is this the correct path to a connection with Gmail - or have I gone 'off-piste'.

David, Please clarify your last bit of advice if you would. My opening statement confims that I have (and use) an established Gmail account with IMAP already enabled in Gmail settings. When I imitially selected 'Add Mail Account' in Tbird 'Account Actions', I had assumed to specify my Gmail account - which resulted in an errorr message So by repeating the same with my existing (Plusnet) Waiitrose.com details I recieved response 'Configuration found by trying common server names' (as the above image). Hence my uncertainty.

But now, having repeated the process for Gmail, I have the resulting Tbird Setings message: 'Confirmation found in Mozzilla ISP database'. Does that mean I have successfully setup the required connection? But no additional (Gmail) folders have yet appeared in Thunderbird. Am I nearly there?

Gmail creates a unique password for you and the pc and validates your account during setup by asking that you enter gmail password into their login screen.

Further progress: What I did found to my horrror upon later trying to access (desktop) Gmail, was that it again failed to open with the message 'No Account Found'. But by following various Recovery pages in Google Help, I noticed a link inviting 'Switch on Cookies' (which ones I don't know) which gave me immediate and full Gmail access. Which was confirmed by their message to Waitrose: 'Account recovered successfully'.

More info:

I have just tried to open Gmail again. Only to be told (again!) that there is no such account. , So now I'm in a closed loop.  And hte only effect I have on web-cookies as far as I know, is by selectively employing DuckDuchGo in Edge, other than when all browisng cookies might be essential. Any ideas?

you really can not set up an account in Thunderbird for gmail without cookies being enabled in Thunderbird. Likewise the entire Google suite of apps and offerings is linked together with cookies. Again you must allow them in your browser.

That's helpful Matt. I do prefer to limit Microsoft, Edge intrusions where possible. So at the moment, with Tracking prevention' set to 'Balanced' and accessing my 'Settings - Privacy, search and services / Clear browsing data on close' options, I have all options active (ON).

So next, I modified the 'Cookies and other data - DON'T clear' setting to include 'google.com' and 'thunderbird.net'. But once again, on reviewing the Tbird (accepted) profile for Gmail, access to my Gmail is still declined.

However, I note that Google insist that in Chrome, 'Cookies and site data' should be set to 'Allow ALL cookies'. So I tried that in Edge and Rebooted, but with the same negative result. Although a direct sign-in to Gmail is still successful in normal circumstances. There must be something obvious somewhere.

Attn Matt: BTW I noticed that OAuth 2 was mentioned as a requirement of the linking process in one of the Tbird/Gmail setup guides. My Tbird OAuth wasn't, so I activated it. Still no luck. Please instruct me to de-activating it again if nescessary. Thanks.

Er . . . Oops - I just found out why. Duly deactivated.

I'm now convinced that nothing is wrong except the information I provided to connect Thunderbird with my Gmail account. So is there anything in the long 'Troubleshooting Information' list that can reset those paarticular settings to default and allow me try again??

Have you gone to GOOGLE-SECURITY and checked if access to 3rd party is enabled?

I've used Thunderbird for years, but about a month ago Google decided to disable my 3rd party access and I had to go in and add Thunderbird again.