More than 1.5 million Firefox add-ons are downloaded every day from our official gallery. Firefox Aurora will allow users to opt in to third-party-installed add-on installation. Users must opt in when Firefox prompts them to install an add-in-on. Firefox will disable add-ins installed by another application unless the user opts to keep them. Users will be asked to opt-in to keep some of the add-addons they’ve installed from within Firefox will be kept, but users will also be asked if they want them.”]
Source: https://blog.mozilla.org/addons/2011/08/11/strengthening-user-control-of-add-ons/