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.
Honestly, they probably store certain config in the local/mozilla folders or whatever and Firefox also does and thus was signed in. Though, this is all speculation.