AFAIK this isn't correct. FDE has some about:config flags set to different defaults than the Stable release, which could mean that a user of Stable might need to flip a switch to allow loading unsigned addons, but there's nothing stopping you from doing so (and addons aren't exactly part of the web platform).
Unsigned addons cannot be enabled on beta or release. That was only available for a limited time. Now they can only be enabled on alpha (aka developer), nightly, or special unbranded builds.
You can make a userscript without any of that hassle, though.
