The separation of "Firefox" (brand, UI, browser history, password sync, extensions, etc) from "Gecko" (an implementation of a rendering engine) seems like a good idea to me, even if it was forced by Apple.

Gecko stands for nothing; it's just code. Whereas the Mozilla / Firefox brand stands for things like privacy, security, etc. (https://www.mozilla.org/en-US/about/manifesto/)

The Firefox/Mozilla brand shouldn't be tied to Gecko or any other specific piece of code.

This is not about code, but about ability to compete.

First of all I'm a convinced Firefox user, I'm typing this in Firefox and I use Firefox on my Android as well and its absence has been one of my main gripes with the iPhone. I think this is great for us Firefox users. But browsers compete primarily in speed and supported open-standards. And even though I'm not one to fall often for the average Joe fallacy, I believe on average people don't give a shit about privacy, otherwise they wouldn't be using Windows or Google Search or Facebook. Of course, that doesn't mean companies or organizations don't have a responsibility to provide privacy or security, quite the contrary.

But if Firefox is just a shell that means (a) absolutely no extensions support, (b) in terms of security or privacy it will be as good or as bad as Safari Mobile is and (c) they won't be able to compete either on speed or on open standards and note that Apple is doing a shitty job in supporting new web standards lately.

They were actually right to be reluctant in supporting iOS, because the only value of this Firefox is providing Sync for us existing Firefox users. Which has been Google's strategy with Chrome as well. Not bad, but lets see it for what it is.

Speed and supported standards were once how browsers competed. Now, features like Sync and a handful of extensions are what bind me to Firefox.

Having Sync available on mobile, even if it doesn't use Gecko or Spidermonkey, is valuable.

Yes, Sync is valuable for me as well. But it will have no extensions either, as those aren't allowed by Apple's policies.

What kind of extensions would you like to use on iOS. Can you give some concrete examples of things (either existing or wishes) you would like to see?

I enjoy the Pocket integration in Firefox, as well as reader mode (which I think is powered by Pocket?).

I also have an extension that allows me to mimic other user agents (useful for some sites to immitate an iPhone on Android). Since Firefox comes with "Request desktop site" by default though, this one is less important.

Other than that: Greasemonkey is very useful to me, as I have a bunch of scripts that minimize page layouts of sites I frequent.