OK so this just happened to me. I have a Mozilla account since I was using their Monitor and VPN service. Last year I canceled those subscriptions because it wasn’t really working for me. Today I start up Firefox and I was signed in to my Mozilla account! I never signed in through Firefox. They’re sus now.
Firefox Sync is end-to-end-encrypted. Mozilla cannot see your synced data.
Either way, I have no idea how this is supposed to happen without you entering your credentials into Firefox at some point. Even if Mozilla wanted to be sus, they couldn’t just guess which account is yours.