The question why IE displays everything has already been answerd in this topic. Besides those badly written websites Opera works JUST fine. My post was biased, true, but so is yours. You want to securely get to any webpage? Securely? If IE is so secure why are updates released so often?
Yes, i can't speak from experience about FireFox but i can about IE and Opera.
In fact Opera is more secure than IE (FF too as far as i know). I can't make my IE settings for it to be secure, you'd say. Maybe, maybe not. If I want to I'll simply search Google and find what and where must be tweaked. The question is why should I when a safer alternative already exists. And it's more convenient as well.
Don't tell me IE is safe with its default settings. That would be a joke.