If they didn't get paid, why on earth did they do it?

Not all incentives are direct financial incentives. It's free marketing. It increases market penetration. It encourages people who only use Chrome to try Firefox.

It was absolutely the wrong thing to do, but it's not like they stood nothing to gain from it.

Is this based on any fact or is it hyperbole?

I don't thing that word means what you think it means.

I'm just asking. It seems like a question that has an actual answer. Someone did it. Someone knows why it happened.

Someone actually that it was fun and valuable. Shocking, I know.

^thought

Of all possible explanations, that is the one I suspect is most divorced from reality.

People who wanted to solve the Mr. Robot ARG/puzzle had to download Firefox to complete the part of it the add-on enabled.

I think it was the opposite of paid. They wanted to draw the Mr. Robot crowd to Firefox.

for the lulz?

Hackers gonna hack. In the “let’s build cool stuff because we can” way.

It seems they wanted press. Well, they got it...

and lost a lot of trust as a result.

They assumed their user base has dwindled to only the kind of anticorporate hacker wannabes that watch Mr. Robot.