As you may know, Firefox 40 has been released today, bringing fixes and new features. Starting with Firefox 41, the users will be able to deactivate the support for unsigned extensions. This features will be enabled by default on FFX42 and newer.
This is a safety precaution, the signed extensions being either checked automatically by testers or verified by humans.
Mozilla took this decision because it was not pleased with the fact that a big number of developers are overwriting the user preferences, change homepages and redirects users to malware.
Google did the same thing with their Chrome browser, the main difference being that the Chrome extensions cannot be installed from external sources.