Many people now have firefox that do not have a clue about W3C standards, etc. That's agreed.
I'm one of the ones on here that pushes firefox, and for good reason. I am a web developer, and have been using Firefox since it was just a little baby (Mozilla M2), simply because I care about and write code that follows the W3C's standards.
All these sites that do not work with firefox are, as vehelpfulguy explained, due to the authors of that site writing bad code that only works in Internet Explorer. The authors of firefox can't do anything about this, but as more people switch over to firefox, the number of IE-only sites will reduce.
Incidentally, other browsers are also affected by this, such as Opera, Safari, Omniweb, Konquerer, etc. The all design their browsers to follow the standards too, by the way. It's just Microsoft that don't. They've improved some things with IE7, but it's still far from being as good as Firefox or Opera. And guess what? The nice guys at Microsoft have also made sure that some sites now that work in IE6 won't work too great in IE7.
Avant only works fine because it's just IE with some extras. It's still no more safe (in terms of security holes) than IE itself is. Other browsers however, are a whole lot more safe.
saxy_jag: there is no more formatting on here because the editors removed it. Simple HTML works fine though.