This is my copy of shumway, signed by mozilla on August 5 2016
It's possible that firefox in the future (or currently) expires signatures on self-hosted addons and will want a newly signed version of the addon--I will not be providing this, as I no longer use it, and I had the mozilla tools installed in a throwaway virtual environment. This impression mostly comes from this post on mozilla support.
If you don't trust my source, you should be able to unzip the xpi file and compare the contents to the official version (this may require building it yourself, I'm not sure).
I have no idea why my xpi is 66mb and http://mozilla.github.io/shumway/extension/firefox/shumway.xpi is only 558kb.