Popular YouTube Unblocker extension hosted on AMO has been blocked and removed by Mozilla. The extension which lets you watch blocked videos on YouTube via Proxy in Firefox browser, once installed, has been found to install a third-party unsigned add-on from developer website by toggling xpinstall.signatures.required preference value to false in about:config without user consent.
To save users from these sorts of situations, Mozilla launched signing for add-ons, and made mandatory from Firefox 46 onwards, this version doesn’t allow install of unsigned add-ons with no override.
Firefox users who’ve already installed the extension gets hard disabled and blocked from being used. The extension in question has been already removed by company from the add-ons site.
“The add-on has a mechanism that updates certain configuration files from the developer’s website. This mechanism has a vulnerability that is being exploited through this website (unblocker.yt) to change security settings in Firefox and install malicious add-ons.”
Read this bug for more details.