I think it's about domains of control: I expect that Google controls which image appears on their home page; I expect that I control which add-ons are installed in my Firefox.

If this code had been in core-Firefox, we'd never have noticed it. Counter-intuitively, maybe it wouldn't have felt as invasive, because I know that Mozilla controls core-Firefox, not me. (And I choose to defer to their judgement, because my other options are to defer to Google or Apple.)