Is there any reason for using Firefox on iOS as opposed to Safari? I'm guessing sync'ing history and stuff with the desktop version; but apart form that?
I hope that there are other things that make a product unique. Like features or defaults or what it looks like.
Now I don't know about possibility of implementing browser plugins. I guess that it should be possible, but API would be different from other platforms.
Apps like Feedly, or Firefox, will have to do additional work to include _their own_ content blocking.
Firefox doesn't use SFSafariViewController. It uses the WKWebView (?). The WKWebView gives the hosting app a lot more control over the webview and whatever you do in the webview is accessible to the hosting app. Cookies, bookmarks, etc. are not available to the hosting app.
I know what Firefox for iOS uses.