thank you for sharing your experiences and thoughts. I agree that it seems low priority for Mozilla. People opening and closing accounts are basic use cases though and in my opinion it’s careless to release such an unfinished system to the public.
I also thought about creating a new Mozilla Matrix ID without a Firefox account and with an e-mail alias instead, but I fear making the situation worse than it already is.
For anyone still trying to find a way to sign up via SSO: Closing your Firefox account and creating a new one with the same e-mail address (which works just fine on its own) has no effect on SSO; the system will recognize your e-mail and won’t let you create a new Mozilla Matrix ID.
For any developer reading this: Please clean the SSO database, if such thing exists! You’re effectively collecting e-mail addresses of deleted Matrix IDs, which is not okay (there is a right to be forgotten). Please remove those e-mail addresses so signing up with different IDs becomes possible!