Since there's likely to be people that do not consider this sufficient, a question out of personal interest: what could Mozilla do/have done to sufficiently prevent something like this from happening in the future?
I'm not sure they could have done anything more short of going back in time and reversing their stupid decision.
It's important not to forgive Mozilla, and to constantly remind them of CliqZ, the tracking in Firefox Focus (also using Google Analytics) and about:addons Discovery page, and of Mr Robot.
Only then, maybe, will they learn something from this. For now, it looks like they don't give a shit.
Come up with an idea of "browser neutrality".
And what should be their source of funding?
Has the idea that Mozilla won't survive without advertising income been substantiated somewhere? (This ad was unpaid, though.)
So I agree that it was not neutral, because there was more of a barrier to other advertisers than to Mr. Roboto. The right thing would be to have hosted the addon on addons.mozilla.org, same as for anyone else.
But it's not a dramatically uneven playing field, given that an open conduit does exist and the reception of any actual advertising was opt-in. (I'm not saying it was ok; it wasn't. But it's not as black and white as you are saying.)
As is I trust they won't use SHIELD next time (and have opted back into their studies based on the policy change), but my expectation is that next time they want to ship some garbage like that they'll use some other channel and marketing have probably already identified it.
The arguments will be over whether sufficient contrition has been shown and whether someone needs to be placed in the stocks.
It was a bad, bad mistake. I think they've learned their lesson.
So simple. So obvious. So necessary, if you value privacy.
Yet they have not acknowledged the mistake or shown any signs of willing to listen to the complaints - their entire explanation is about something else that people weren't even aware of!
Given the level of feedback and the intelligence of the recipients, this is unlikely to be a mistake or a coincidence. It is reasonable to conclude they fully intend to repeat this behaviour in future.