That doesn't appear to be correct. All unsigned extensions will be blocked in all release versions. Without a developer/unbranded/nightly version, you're not allowed to install anything that didn't come from Mozilla's add-ons site. https://news.ycombinator.com/item?id=21418604
Yes, but you can sign your xpi via a command line tool (which calls the AMO API) and then distribute that however you want. It can be installed by dragging into about:addons.
