Firefox 45 or later, stable or beta versions allow developers to test add-ons that were not signed by Mozilla. From Firefox 44 onwards, stable and beta versions of Firefox won’t allow the install of unsigned add-ons, the preference xpiinstall.signatures.required, which, allowed unsigned extensions to be installed in Firefox 43 won’t be available to use, Mozilla will remove it.

However, early adopters using Nightly or developer, can still able to flip that preference value to false and use non-signed add-ons without an issue.

Keeping add-on developers in mind, starting from Firefox 45,  Mozilla will allow them to test unsigned restartless add-ons temporarily for a session. For those interested, to load add-ons temporarily in Firefox

1. Visit  Firefox’s new about:debugging page

2. Click on Load Temporary add-on and select the XPI file for the add-on

Firefox about debugging page with load temporary add-on option

3. Add-on then will be loaded and work in that browser session, when you restart the browser, the add-on will no longer  be loaded, to make it work again, you need to re-load it by visiting add-ons manager again.

temporary add-on loaded