In Abrowser's 'Preferences' menu, the 'Security' tab lists the following websites which has permission to install addons, without issuing any warnings to user, Under 'Exceptions...' button. (See the attached image.)
[1] addons.mozilla.org
[2] marketplace.firefox.com
These websites host non-free addons/plugins along with the free counterpart. Remove these websites and replace them with websites that suggest exclusively free addons/plugins.
Yes, addons from the Abrowser Addons site are actually downloaded from those servers. 'Suggestion' or 'steering towards' is more than just using the name. This is working as designed.