Then why doesn't Mozilla include it in the installation of Firefox? That's one of my beefs about FF. If you install it just in it's default state with no addons, it is less secure than IE8 is in it's default installation. Users should not have to look for addons and plugins to secure their browser, they should be included by default. I wouldn't use the addon anyway probably because I wouldn't like dealing with having to manually allow parts of web pages to load. I want to see everything on the page. I don't even block ads , only popups and cookies which I allow selectively by site.
There are other things about FF that I don't like such as the download interface and the cookies management where I think IE8 is far better. I also have never found it to be any faster.
IE8 is a piece of crap, period, if you can't see it you got a problem, or problems

Why Firefox doesn't include NS, well first because the NS developer doesn't work for Mozilla, and second even if there was an agreement
most IE users like you would get lost with NS when trying Firefox.
As to IE being more secure than FF default config, what are you smoking

FF doesn't write entries to the registry while browsing, FF does't use these freaking activeX things, one of the biggest source of potential malware on the internet,
interacting directly with your operating system.
Rare silently installed extensions in Firefox (previous versions >>> 2.0) from bad sites (became impossible now btw, while browsing), didn't affect the OS, just the browser.
There's something that I hat more than IE in this world, it's the guys promoting it and talking BS about Firefox. Firefox was, is, and will be the more secure browser of all times. Now ****, thanks.
ps:on a side note, FF might be currently twice slower than Chrome, but guess what, IE8 is twice slower than FF

as to the interface if Internet Explorer, it's hardly better than what it was 10 years ago, everything in it is completely outdated, the whole interface is a failure, favorites, downloads, settings etc...
@ the others: I'm a long time NS user >>>
NS is not for everyone, that's a fact. Live with it. Put NS in the hands of an average user is to some extent like doing the same with a HIPS (to some extent...). They wouldn't know what to answer.