I would agree with you, if the add-on in question was not developed, shipped, and offered by the people who made the browser, of which the add-on sandbox is a part.
In my view, that sandbox is a trusted area between the browser and the user.
Mozilla has the privilege accorded to it as the developer of the browser, to modify the addon sandbox so long as they don't infringe on my interests, e.g., security, stability, privacy, speed.
For example, Chrome automatically disable extensions that ask for too many new permissions upon update. Chrome will also make it difficult to add extensions that are not listed on the chrome store.
If we remove the right for browser developers to install, uninstall and alter add-ons, then we're essentially forcing them to modify the browser instead, which is overkill for the add-on in question.
At the end of the day, if you can't trust the developers of your browser, then you should install another one and disable add-ons entirely.
When you install Firefox it asks whether you want to take part in these studies. You can also change the setting at any time in preferences (about:preferences#privacy section).
This isn't about what the addon itself does or does not do, it's the principle of force-pushing unwanted content without prior affirmative consent.
This would apply even if the addon was just a stub that didn't have any executable code in it. In this case, it's worse: an ad.