Just guessing here, but I strongly suspect that by "performance" they're only paying attention to the number of people who made Firefox the default browser right away and totally ignoring the number of people who were so irritated that they won't use Firefox again, ever, unless forced to do so.
It's like the websites that justify throwing up a million interstitial popups by claiming that it increases mailing list signups by 0.012%, while blithely ignoring all the people who are angered by such time-wasting shenanigans.
You're allowed to ask me to install your software/subscribe to your newsletter/like your app on the App Store.
Once.
More than that and I'm going to do anything I can to avoid using your product ever again.