I'm not privy to the details, nor have I ever seen Mr Robot. That said, I believe there were hints in the show about using Firefox to solve some mystery. The idea was that users would then go open Firefox (which may have been sitting unused on their machine for years), and then discover that Firefox and Mr Robot were in cahoots. The viewers would presumably find this cool and exciting, but everyone else demonstrably found it creepy.

If I understand correctly, at some point when following the breadcrumbs the user is given the opportunity to opt in to the game. I think everyone now agrees that this opt-in step should have triggered the download and installation of the add-on, rather than the activation of a dormant add-on that was deployed to every single Firefox user.