Mozilla’s Firefox add-ons platform has traditionally been very open to developers. This gives developers great power and flexibility, but it also gives bad actors too much freedom to take advantage of our users. To keep this balance, Mozilla has come up with extension signing, which will give us better oversight. Extensions that arent hosted on AMO will have to be submitted to AMO for signing. There will be a transition period of two release cycles (12 weeks total) during which unsigned extensions will only generate a warning.”]
Source: https://blog.mozilla.org/addons/2015/02/10/extension-signing-safer-experience/