I don't think it is necessary in this case for Mozilla to even be in possession of transiently-non-aggregate data, though. If they want to analyze click-through rates or whatever, they can have URLs that are constant across users and not lose any data. For credit cards, the payment processor must have the unique card information, so collection is warranted in that case, even if they are never stored.

That is: since I have no insight into what happens to the data once it reaches their servers (outsourced to an external analytics platform), I object to them collecting it unnecessarily in the first place. Storage does not come into play, since while I trust Mozilla I don't trust the third-party, no matter how much Mozilla claims to have vetted them.