This feature has landed and has been in production for a while. I think this is safe to close and we can file follow up issues.

I am getting the error message "Primary account email required for sign-in" when I attempt to sign in with my FIrefox Sync account. A screenshot can be seen here: https://i.imgur.com/RAyNPGd.png.

My account email has a plus sign in the local part (refer to https://en.wikipedia.org/wiki/Email_address#Sub-addressing). But that is the email address I used to set up Firefox Sync, before any of the other cloud services came along. If I try to remove the plus-addressed part, I'm led through the user flow for creating a new account.

Is this this bug, or some other bug?

But that is the email address I used to set up Firefox Sync, before any of the other cloud services came along.

This error message occurs if you have changed the primary email address on your account, typically when you are trying to use the old primary address to sign in rather than the new primary address. Did you change the primary email address on your account?

I recommend filing a new bug at https://bugzilla.mozilla.org/enter_bug.cgi?product=Cloud%20Services&component=Server:%20Firefox%20Accounts to discuss, because if necessary we can mark the bug confidential in order to investigate the specific details of your account.

I was able to reproduce. The issue is that the cached FxA login screen is showing the new primary email, but actually supplying the old one to the server… clicking "use a different account" and retyping the email address is the quick fix.
https://github.com/mozilla/fxa/issues/3427

Nice find :rfeeley! I'm not entirely sure what the needinfo? is for but it seems like we should be able to fix it with a server-side FxA change.