Sadly it's actually the other way around. Those webpages are more compatible with IE because they are poorly written. IE has generally been very poor when it comes to adhering and supporting the various web standards that exist.
Basically what happens is that Firefox and other good browsers expect web documents to adhere to these standards and when they don't the resulting page will usually not look as intended. IE however doesn't really care and pretty much attempts to display anything no matter how broken the actual code is and also deviates from or contradicts the standards in many areas, causing web pages that were written and tested specifically for IE (even if they don't contain any sloppy coding) to look bad on browsers like Firefox.
I've worked at a web development company (thankfully mostly doing back-end development unless we were really low on staff capacity) and have witnessed first hand the joys of getting complex websites to look good (or even just "the same") in both browsers. That is why IE is not exactly popular among web developers.