https://blog.mozilla.org/addons/2016/01/22/add-on-signing-update/
We’re adding a feature in Firefox 45 that allows temporarily loading
unsigned restartless add-ons in release, which will allow developers of those
add-ons to use Firefox for testing, and we’d like this option to be
available when we remove the preference. We also want to ensure that
developers have adequate time to finish the transition to signed add-ons.