Miren, Mozilla decidió cometer suicidio:
https://blog.mozilla.org/addons/2015/08/21/the-future-of-developing-firefox-add-ons/

>Firefox 42 and beyond:
>>The Beta and Release versions of Firefox based on 42 and above (Beta 42 will be released at the same time as Firefox 41) will remove the preference that allows unsigned extensions to be installed, and will disable and/or prevent the installation of unsigned extensions.

>The strategy announced here necessarily involves a lot of trade-offs. Developers who already support Chrome extensions will benefit since they will have one codebase to support instead of two. Developers of Firefox-only add-ons will have to make changes. Those changes may require considerable development effort up-front, but we feel the end result will be worth that effort for both Firefox’s users and developers.

Ahora sí que intentan ser Chrome. Buen trabajo.

>Es más fácil que nunca personalizar Firefox y hacerlo funcionar de la manera en que trabajas.
>Ningún otro navegador te da tantas opciones y flexibilidad.
(risas)