As a follow up I checked FF of the second user on a separate machine and the usernames and passwords had been immediately imported there.

I've now the deleted usernames and passwords from FF for the second user on the other machine and I have deleted the new user W10 account from on the pc.

How can this be prevented ? Worrying . . .

I was worried that it may have deleted my usernames and passwords but thankfully it did not.

...to my surprise when I opened Firefox and logged in to sync with the new username...

I’m not very familiar with Sync but you do realize that the user/password combination you used for signing in to Sync from Firefox is unrelated to user accounts on PCs, don’t you?

In other words, any user in the world could set up Sync on a Firefox copy and sync to your copy as long as they use those same user/pass credentials for Sync, if they had access to it. User accounts on Windows aren’t similar. If you don’t want this to happen, do not sign in to Sync with the same credentials for that user account on Windows, but set up Sync for a different email, and only if you (or that user) need(s) it - it is not a requirement to do so.

Kimberl said

I have created a second user on my pc and to my surprise when I opened Firefox and logged in to sync with the new username the usernames and passwords from the original user are in the second users username and password list. How can this possibly happen ?

Same Firefox Account as used with the first LUA?

LUA=Logon User Account as it's called in Windows

I’m not very familiar with Sync but you do realize that the user/password combination you used for signing in to Sync from Firefox is unrelated to user accounts on PCs, don’t you? Yes

In other words, any user in the world could set up Sync on a Firefox copy and sync to your copy as long as they use those same user/pass credentials for Sync, if they had access to it. User accounts on Windows aren’t similar. If you don’t want this to happen, do not sign in to Sync with the same credentials for that user account on Windows, but set up Sync for a different email, and only if you (or that user) need(s) it - it is not a requirement to do so.

Same Firefox Account as used with the first LUA?

Totally different user names and passwords.

I hope I have now demonstrated some IT competence here and we can move forward with some constructive thoughts.

If you used the same email address to connect to Sync then data stored on the Sync server will be synced to the current profile folder. A Sync account is identified by the email address. You would have to use another email address to keep sync data separate.

Sorry I didn't intend to insult you.

This statement had me confused - "logged in to sync with the new username the usernames and passwords from the original user are in the second users username and password list." The simple Password that a user chooses for setting up their Firefox Account (Sync account) isn't used over the internet; that Password is used to generate a a 64-character "password" algorithmically within Firefox based upon what the user chose as their Account password. And the chances that Firefox would do that "again" is infinitesimally remote, all but near impossible. Plus that's done twice for the kA and the kB keys; one of which is used to encrypt the data going to the Sync server and the other for the login at the Sync server.

And this response is way off - "In other words, any user in the world could set up Sync on a Firefox copy and sync to your copy as long as they use those same user/pass credentials for Sync, if they had access to it."

Kimberl said

I hope I have now demonstrated some IT competence here and we can move forward with some constructive thoughts.

Not from me! I am not partial to insults.