I just know that when testing and browsing standards-compliant sites, things tend to break more in Mac IE 5 and Opera the most.
Well there you go. I never understand why Mac users would want to use IE.
And I assure you, if anyone of the other browsers was used as commonly as Windows IE, we'd be finding out about exploits and security holes in them as well. Here's
an article about a Mozilla/Firefox vulnerability that's popped up.
I'm not saying that Microsoft's got the best security coding or anything, but when you're number 1 in use, the hacker community is going to find (and most likely exploit) your product.
As for web standards and stuff, I agree, there's issues...but then again there's always going to be issues as long as we've got so many competing products. It would be great if all the browser makers would just create a common rendering engine, and then distinguish themselves with their shell, but I'd imagine that won't ever happen, at least not in my lifetime.