Plus users do not have an alternative, at least on iOS. They all pass through webkit.
On a desktop, if Firefox breaks, people will just switch to Chrome or something else. Plenty of people want things to "just work" out of the box.
Maybe the better solution is to have a guided install process. When installing firefox, ask the user if they want it to "just work" or be "privacy conscious"? Warn them of the tradeoffs of both and let them decide once.
Safari has enough market share that web developers must work within Apple's guidelines if they want their websites to work on iOS.