Mozilla has begun the process of requiring all add-ons for the browser to be signed. The goal for the change in policy is to protect users from malicious extensions. Mozilla also has expanded its warnings about malicious and unwanted software in Firefox 40. Google has taken the approach of only allowing developers to distribute extensions through its Chrome Web Store. Starting with Firefox 42, signatures will be enforced on all extensions in both the beta and release versions of the browser. The next version of Firefox will have an option that allows users to enforce signatures, but users will be able to set the preference.
Source: https://threatpost.com/firefox-40-begins-warning-users-about-unsigned-add-ons/114237/