They are using the WebKit rendering engine, but there’s a lot more to a web browser than just its rendering engine. They aren’t just skins.

Could you add a more to that? What other complex parts are built by FF?

Why the “complex” qualifier? The code responsible for the incorrect favicon URL bug is very simple, and part of the Firefox codebase not WebKit:

https://github.com/mozilla-mobile/firefox-ios/blob/bd589c194...

Firefox on iOS has about 200k lines of Swift. As a rough rule of thumb, everything that isn’t directly related to rendering something within the page or executing JavaScript is Firefox code not WebKit code. So bookmarks, syncing, tabs, etc.

Not necessarily complex, but the tab switcher etc is built by Firefox, and probably is what tries to load the favicons to display the icon somewhere?

What do you think is complex?

Yea, that's basically it on iOS, although there is a bunch ux, so it's not nothing

On android, browsers can ship their own engines, and they do. There is actually some freedom on android.

This changed very recently due to EU rules