It seems to me that the account settings in Thunderbird 140.6.0esr (64-bit) have changed,

Nope

because without any supporting documentation in 'help' I can no longer see a way to enter a password.

See https://support.mozilla.org/en-US/kb/password-manager-remember-delete-change-tb which you will note has not been edited since September last year and that was a minor edit.

The current version is expanded from my original blog post in 2011 https://thunderbirdtweaks.blogspot.com/2011/07/pssswords.html with the addition of an option to edit a saved password. But I can not see there ever being a need to allow the user to enter a password there.

So you are looking for something that has really never existed. Thunderbird will ask you for a password if it needs one and offer to save it.

However just setting the authentication method to password does not guarantee you will be asked if you have other software on your system that consumes the connection attempt so the server never requests a password. Common causes of failures to request passwords are things like antivirus products and in your case they are also blocking access to localhost and the odd proxy type port numbers that Proton uses.

Port 1143 appears to be registered for use by informix to exchange data. See https://www.infomatrix.ro/ and https://isc.sans.edu/data/port/1143 Port 1025 is often used as an alternative SMTP port, but it has a past in the Windows RPC and the associated vulnerabilities and patches over the years so a problematic choice on most occasions. See https://www.pentestpad.com/port-exploit/port-1025-microsoft-rpc-microsoft-remote-procedure-call

I have two gmail accounts which are working but I also have a Protonmail account which has stopped working. The Protonmail Bridge show the following details for entry in the Thunderbird client's account settings: IMAP: Hostname (127.0.0.1), Port(1143), Username(withheld), Password(withheld), Security (STARTTLS) SMTP: Hostname (127.0.0.1), Port(1025), Username(withheld), Password(withheld), Security (STARTTLS) I can enter the following: Hostname, (127.0.0.1), Port(1025), Username(withheld), Connection Security(STARTTLS) & Authentication method (normal password) but nowhere to enter the actual password!

You enter it when you are asked for it. Always have. Thunderbird does ask you to enter it when you are first creating an account, simply to reduce the number of password requests as there may be many attempts to connect to the mail server in the setup process attempting to detect the servers settings.

How do I do this and where in the docs is this described, please Thanks

As you appear to be using Linux in one of the X11 version I will guess it is probably a Ubuntu derived distribution and will be using SNAP packages with their special sandbox built in. How that works with those ports, I have no idea. But many folk have issues with the SNAP versions and permissions and have reverted to DEB packages which just work.

Ubuntu derived distributions will also most likely have the Ports 1025 and 1143 closed. While the definition of localhost says that the transmission should never go to a physical adaptor, I could see it being bundled with the general software firewall rules. So have you checked your software firewall? Have you checked your installed version of Thunderbird has access to those ports through the snap sandbox.

Do you have development environments or server software like Apache installed that may by consuming port 1025 connections or even things like mailpit or mailcatcher?

I'm sure I'm not going mad/senile. I remember entering my Protonmail password!? Also Wayland/OpenSuSE

Could my partial configuration of Nginx/LetsEncrypt/PiHole have a bearing?

Err this attachment happened this evening - I didn't think I'd done anything with a SS Cert

I'm sure I'm not going mad/senile. I remember entering my Protonmail password!? Also Wayland/OpenSuSE Could my partial configuration of Nginx/LetsEncrypt/PiHole have a bearing?

As I said before when you configure your account initially you are asked for your password. Also as I said you were asked for it and if it is needed again, or if the saved password is unsuccessful in making a connection, you will be asked for it again. After that it is saved and you can access that from the password manager.

Why are you using encrypted communications on your own hardware? the data travels nowhere, but you are adding the overhead of encrypting the communications between two software items on your local machine. I am sure an argument can be made that it makes the interception of the communication harder, but in all honesty Thunderbird stores your mail as plain text, so it is some weird transitory level of security. However you will need to add an exception to the certificate in the certificate manager is you want to use it, or use a trusted certificate issuing authority to sign your certificates.

But you are at point 4 or 5 in the Proton setup https://proton.me/support/protonmail-bridge-clients-windows-thunderbird

The dialog is different but the root cause is largely the same. Use the certificate manager to ad an exception for the server. (search cert in settings)