Why not just look at market share across all devices? Which makes chrisseaton's point: Firefox sits at 3.26% as opposed to Safari at 18.61%.
“2.5.6 Apps that browse the web must use the appropriate WebKit framework and WebKit Javascript.”
From the App Store guidelines.
It’s not a choice thing.
*without jailbreaking
It sucks that Apple does this but it doesn't actually change the fact that Firefox use is at 3%.