Many people seem to be shocked because Mozilla installed an add-on automatically. In my opinion, it doesn't really matter since the code is coming from Mozilla - they're building the whole browser, so they could introduce functionality anywhere. If someone distrusts their add-ons, why trust their browser at all?
The main question is what behavior is being introduced. I haven't researched deeply, but apparently the add-on does nothing until the user opts-in on studies.
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