In Firefox 43, we made it a default requirement for add-ons to be signed. This requirement can be disabled by toggling a preference that was originally scheduled to be removed in Firefox 44 for release and beta versions (this preference will continue to be available in the Nightly, Developer, and ESR Editions of Firefox for the foreseeable future).
We are delaying the removal of this preference to Firefox 46 for a couple of reasons: 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.
The updated timeline is available on the signing wiki, and you can look up release dates for Firefox versions on the releases wiki. Signing will be mandatory in the beta and release versions of Firefox from 46 onwards, at which point unbranded builds based on beta and release will be provided for testing.