Wild guess:

2.5.6: Apps that browse the web must use the appropriate WebKit framework and WebKit Javascript. -> Firefox is No-No

5.2.5: Apple Products: Don’t create an app that appears confusingly similar to an existing Apple product, interface (e.g. Finder), app (such as the App Store, iTunes Store, or Messages) or advertising theme.

https://developer.apple.com/app-store/review/guidelines/