When something doesn't work perfectly in all the modern browsers these days, it's usually because it's either very poorly designed, or very OLD. Take this forum software for example. I'm guessing it's built upon code that was written at least 10 years ago.
The latest versions of Firefox, Chrome, IE, Safari, Opera etc...are all good. At least, there is nothing wrong with them that should screw up websites that have been designed to modern standards. How they differ, I suspect, is in how they deal with ye ancient code.