Asked an entire room full of webdevs yesterday if any of them knew that FF/Chrome/Opera/Brave/etc. for iOS weren't allowed to compete on engine quality.
Zero hands up.
— Alex Russell (@slightlylate) September 25, 2019
It's worth making this clear then. On iOS, the only browser engine is WebKit. There are other browsers, but they can't bring their own engine (Blink/Gecko). So, if you're using Chrome or Firefox on iOS, it's really the same engine Safari is using, only slightly less capable (e.g. no third-party content blocker apps work in them).
It's worth knowing that as a developer. While Chrome supports stuff like service workers on their desktop browser and on other platforms, the browser engine made available to non-Safari browsers on iOS does not. You don't have them there. Likewise for Firefox.