As I tell everyone that tries to defend this, it's not about what the plugin did or didn't do. It could have literally been a copy of about:blank, it changes nothing.
The entirely justified outrage was its purpose for being put there (which boils down to advertising) and the lack of consent for its being put there. That's it.
Doing nothing is not advertising. The only way for this to be used in advertisement was from Mr. Robot fans to Firefox.
> lack of consent
Pretend it was a copy of about:blank when you answer this question: What makes this different from the giant pile of patches merged into each release of firefox that you don't read?
(I'm making this a separate post because I don't want any distractions in the other one.)
If they had done it correctly, it would have been invisible and it would not have advertised anything to firefox users. That is the purpose. It showing up the way it did was an oversight.
The entirely justified outrage was its purpose for being put there (which boils down to advertising) and the lack of consent for its being put there. That's it.