You are not allowed to submit a web browser to the app store with your own javascript interpreter.
There's no firefox on iOS?
Firefox (and every other 3rd party browser) internally uses the same web rendering engine as Safari. Rolling your own renderer + JS engine is not allowed.
No. But there is a "Firefox" shell around WKWebView.
Apple let’s you take the naked core of safari, stripped of chrome, and slap a skin on top.
All iOS web browsers use a web view backed by Safari.