New profile requires new account, but when I try to log in with a different account, it tells me the primary email must be used

I have a Firefox/Mozilla account associated with one email address. That is my "original profile." When I try to create a new "personal" profile, and I try to log in with a different email address, it tells me that I have to log in using the primary email for my account. But I cannot do that, because I cannot be logged in to two profiles at the same time using the same account.

How can I create a different account for a different profile? I tried doing so, but it doesn't seem to allow me.