From the article, quoting a Firefox dev explaining the decision:

@McCovican @jonny @mathew @RenewedRebecca Opt-in is only meaningful if users can make an informed decision. I think explaining a system like PPA would be a difficult task. And most users complain a lot about these types of interruption.

In my opinion an easily discoverable opt-out option + blog posts and such were the right decision.

puts on They Live glasses

@McCovican @jonny @mathew @RenewedRebecca If we had made it opt in, then not a single human being on the planet would have enabled it, and we didn’t want that