Firefox on iOS doesn't even exist really, it's a lie. It's a Firefox front-end to Safari designed just to look a certain way and sync settings and bookmarks. Apple won't allow anything more, it's not Mozilla's fault.

Is the only thing that defines a browser its underlying rendering engine?

I hope that there are other things that make a product unique. Like features or defaults or what it looks like.

okay it's probably fair to say "it' snot a lie" but it's definitely not what I expected (a full build of Firefox for iOS).

I wonder if it'll eventually be possible to compile Firefox to WASM and then run it on iOS Safari...dawg...

It does not; Apple does not allow code execution outside of its app store.

Apple allows limited code execution. You can build cordova application and download new JavaScript code as an update. That's fine from Apple's PoV. They don't allow arbitrary machine code execution, like someone getting into AppStore and then downloading and executing malicious code calling private API. But if your code is run with JavaScript engine, it's fine.

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.

iOS has built in content blocking that works with Safari and the newest embedded web view - meaning that you get the same ad blocking in apps like Feedly.

Unfortunately that is not true. The native content blockers that you can install for Safari will only work in Safari.

Apps like Feedly, or Firefox, will have to do additional work to include _their own_ content blocking.

There are multiple ways to embed a webview in an app. The SFSafariViewController is an out of process webview that you can embed in your app. It can share cookies with Safari and it will use whatever content blocker you have installed. The hosting app doesn't have any access to the cookies or any information you type in. Feedly uses this web view. https://stackoverflow.com/questions/32681511/deactive-ios-9-...

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.

Ah yes I see now that Feedly has an "Open Webpage Directly" option. I've never used that before since I prefer the Feedly rendered version.

I know what Firefox for iOS uses.