Well, for example, if it ran Gecko like it does on Android, you'd be one step closer to running extensions like uBlock Origin that run fine over on the Android side of the fence. Similarly, you might be able to use a password manager extension (e.g. LastPass or Bitwarden) that doesn't support the full suite of features that Apple gives its own password manager (e.g. updating a stored password based on an updated entry--and yes, I COULD just use Firefox's password manager, but maybe my usage of password spans beyond the browser or even THAT browser specifically).

And also you might care because we're on a forum full of developers knee deep in a thread and walled gardens and their relation to customer architectures, and it might be relevant to that thread that code that's common to every other Firefox platform target isn't as common on iOS.

I didn't say you should care.

If you used Firefox extensions - as many others do - then you probably would.

Sorry no idea at all what you're saying here. If you know what WebKit is then you probably know what rendering means.