Forcefully installing ads, or enabling plugins is a big no-no, and a company like Mozilla should be aware of the implications of its actions.
While advertising is a given in todays day and age, when it comes to the authority or actions of software, ultimately the user should be able to decide.
It is perhaps why programmes like little snitch are becoming more popular as a way to alert users when applications are performing unusual tasks, or sending data to unknown locations.
It is unclear what the add-on did, as most users disabled and removed it from Firefox right away. Looking at its source code, available on GitHub, the add-on was supposed to trigger only on three very obscure URLs. "https://www.red-wheelbarrow.com/forkids/*", "https://www.whatismybrowser.com/detect/*", "https://red-wheelbarrow-stage.apps.nbcuni.com/forkids/activitysheet/" Mozilla force-installed this add-on via Firefox Studies, a Firefox feature that allows Mozilla to run experiments in users' browsers. According to a test carried out today by Bleeping Computer, the Firefox Studies feature is turned on by default for all new Firefox installs.