Thank you for reporting this.

I think the existing account in the "Set up your existing email address" means that you should already have a working email address to add a new account in Thunderbird.

Thunderbird will not create an new email address for you, it just uses your existing email to setup a so-called account in its user interface so that you can use it.

Do you agree?

So you mean I have a working account somewhere else and I want to make a new Thunderbird account? No, I have a working Thunderbird account and I want to add a new account. When it says "Add a working account", that's the account that I already have. Where does the new account come it to this?

For me,

menu - new account - email

will display a new account setup interface, and the "Full Name" field is filled with an existing account name. You can just change that and follow the steps.

Do you have different interfaces?

What I'm seeing is nothing like that. There is no menu - new account - email.

In the new account setup screen, the "Full Name" field is not filled with an existing account name, just a "John Doe" frinstance.

The screen strongly suggests that if I enter a new mail service, Firefox will nuke everything that I have. It does NOT say add an additional provider.

I am trying to add a new provider, which means adding a new account.

Did you already create an email address with your new provider? Usually you do this in the web interface of the provider.

When I click "Add new account" it takes me to a page that asks me to enter my existing account.

This page is also called Account Hub which serves to set up a new account.

Does this mean the currently running account or the new account?

This means that you fill in the email address of your new provider. In one of the next steps you'll also be prompted to fill in your password to log in your new account.

In the new account setup screen, the "Full Name" field is not filled with an existing account name, just a "John Doe" frinstance.

This is OK

The screen strongly suggests that if I enter a new mail service, Firefox will nuke everything that I have. It does NOT say add an additional provider.

This has nothing to do with Firefox which is a web browser. Thunderbird is an email client. You don't fill in your new provider's name but your name and your new email address and click "Continue" ... etc Setting up a new email account doesn't wipe out already existing other email accounts.