Welcome aboard.
Now, here's what makes Iron better.
Iron loads up faster, it runs under WebKit, and it doesn't bog down because of extensions (or at least not much, not noticeable to a point).
Those benchmarks, I guess.
I use Palemoon, which is an optimized Firefox. Even with many extensions, Palemoon is still takes a while to load-up.
Where I find it better is that Firefox still loads pages faster, according to my real-life tests, I don't go by those benchmarks because even though the browser is faster, you might not notice it.
Example: Ever tried opening Firefox with and i5 or i7 processor? They open so fast that you don't notice which one is loads up faster. Either way, on my example test, i7 is clearly faster. This should also apply to those Java and etc. benchmarks.
Let's not forget the wide varieties of Add-ons, but it won't be long before Chromium obtains the equivalents.
If you aren't up-to-date, Firefox Aurora has addressed "Browser start-up speed" and "memory consumption." I just use those channels for the Nightly, which I hope to get 9.0 when I update the browser, it's released today. (Haha, I just noticed that you put 6.0 under the test, good that I didn't enter my message before reading your benchmark list) Also, I notice the start-up speed on the 8.0a1 Nightly, the add-ons might still bog it down, yet I only have Ghostery on installed(Ghostery might not work on 9.0a1 for a little while, though).
When I want something really quick to look-up things, I go with Iron. When I want to get serious about what I do, I go with Palemoon(Firefox).