A few more thoughts. Are you trying to use an app password for Gmail instead of your main account password? Are you using 2-factor authentication? If so, what is your 2nd factor? Are you using a passkey for your Google account?

I only have a single Google password which is used to open Gmail from Firefox. If I am signed out of my Google account, when I click on Gmail I am asked to choose which account to open. I select my email account and it opens without further verification. Certain other apps that I use, such as ebay, do use two factor authentication, usually by a code sent to my phone and to my tablet. As I have mentioned before, my Gmail account shows that Thunderbird has permission to access my emails, so the problem definitely seems to be that Gmail will not recognise the link between to two.

Hi Chris (again), Just tried opening my Windows 11 in Safe mode with Networking again and have just realised that there is no internet connection. Surely I need that to try and establish the link to Gmail. If so, how do I establish an internet connection in Safe mode?

Just tried opening my Windows 11 in Safe mode with Networking again and have just realised that there is no internet connection.

Presumably you're connecting to your Internet Router via WLAN. The Windows WLAN driver is disabled in safe mode as well. So you'd need to use a wired connection to your router.

I only have a single Google password which is used to open Gmail from Firefox.

You explicitly mentioned Firefox here. The password you use in Firefox is also the one you should use in Thunderbird for that Gmail account. Is this what you're doing?

when I click on Gmail I am asked to choose which account to open. I select my email account and it opens without further verification.

So is this still what you're seeing when logging into your Gmail account using Firefox? There is no password prompt?

my Gmail account shows that Thunderbird has permission to access my emails

This is when you're logged into your Google account via Firefox, and you're checking your Google account settings? Since you deleted your Gmail account in Thunderbird you can remove the entry for Thunderbird being allowed to access your Gmail account in Google account settings. The permission for Thunderbird to access your Gmail account will be restored once you successfully authenticated to your account using Thunderbird.

Also see https://support.mozilla.org/en-US/questions/1572104 May be you're missing an authentication prompt when attempting to authenticate using Thunderbird?

Hi Chris, I have checked my Google security settings and I do have two step Authentication, the second step being a message to my phone or tablet asking me to confirm that it is me trying to log in to my Gmail account. I have now removed the reference to Thunderbird from my Google settings. The password I use for Gmail is my main Google account one. I usually stay signed in to Google so do not often get the verification message.

If I log into Firefox  when I am signed out of Google and try to log into Gmail, I am asked to select my Gmail account then enter my password. I then go straight into Gmail without further verification.

Unfortunately I cannot try The Safe Mode method with a cable connection as my PC is too far away from the router.

I usually stay signed in to Google so do not often get the verification message.

Are you getting the Google 2FA prompt when you're trying to re-create the Gmail account in Thunderbird?

No, no Google prompts on Thunderbird.