They are skins around Safari. From https://en.wikipedia.org/wiki/Firefox_for_iOS

> Unlike Firefox on Android, Firefox for iOS does not support browser add-ons. Additionally, it uses Apple's Webkit rendering engine, rather than Mozilla's Gecko. Both of these limitations are in accordance with Apple's rules for submitting apps to the App Store.

Search for safari, webkit or WKWebView in https://github.com/mozilla-mobile/firefox-ios and you'll see how the integration goes.

Calling them skins is not correct at all. Being able to auto translate whole pages or sync all bookmarks and browsing history or passwords in iOS chrome is not merely a skin.

Those are both Safari based on iOS. Specifically, they can't use their real rendering engine - they have to use the safari one.

No PWAs