Yep, roughly speaking you can't compete on speed with your own rendering engine because you cannot do Just In Time style memory optimizations in the iOS sandbox.

Only Apple can ship that type of code, so any browser from the App Store competes on features with Safari, not on memory/speed etc.

All browsers on iOS have to use Safari, so Firefox for iOS is just Safari with Firefox's UI and Sync.

It is still true for Firefox. Apple does not permit or approve applications that can execute their own code. Such a thing permits you to have "webapps" that are not under the purview of Apple and the walled garden. As a result, all browsers are just reskinning Apple's browser.

They had to make a small exception for educational programming apps, but that isn't Firefox.