I would disagree with FF being the problem with web page rendering.
If a site renders correctly with IE and not FF, there’s a good chance the site designer was very sloppy coding the site.
It’s very possible to code site that will render properly with IE, yet will not render correctly with FF.
However, I have yet to run across a site that renders correctly in FF, but does not render correctly in IE.
I do most of my dev work in FF and check IE second. There are many times when I see what I want in FF and have to make special changes to see it proper in IE.