iOS requires a browser to use the OS-provided WebKit, but you can still use your own networking layer, and doing your own scripts injection (e.g. for extensions, like what Orion is doing). Firefox for iOS used to use Alamofire as its networking engine, but switched over to NSURLSession/URLSession at some point. Chrome for iOS uses Cronet which was extracted from Chromium's networking stack (or maybe used, I have not followed the development recently).