Which makes them not really third party browsers anymore, they're basically frames around a UIWebView (which is Safari) which can't do anything interesting.

UIWebView and Safari are different things. Yes, Safari uses (essentially) UIWebView, but a browser is much more than its rendering engine.

The important bit there is "do anything interesting". Forget about any plugins that directly manipulate the markup (so no µ/adblock, no ghostery, no noscript).

Yeah, they can display web pages, but iOS Firefox/Chrome provide almost no benefits over the stock safari. Less in fact, since it's a near certainty that Safari is using privileged APIs to integrate with the OS in a way that no other browsers are allowed to.

(Didn't a very prominent company get sued for something along these lines back in the 90s?)

You can actually load whatever you want into a UIWebView. You could do the request, manipulate the response and display your modified HTML.

Adblocking is of course now integrated into the OS.