The signing requirement only applies to the stable and beta versions of Firefox. It can be disabled in about:config in Firefox Developer Edition (which is basically the same as beta), as well as in Nightly, ESR, and Unbranded builds [1].
this is not true. with Android version at least, an AMO account is also required.
I was referring to desktop, not Android here. To my knowledge, Android currently requires a custom collection on AMO for installing anything outside the default list.