This is a problem with sloppy developers, not the browser. Sloppy developers with non-standard code break Firefox. IE is like web browsers for dummies because it accepts broken code and tries its best to fix it. Most of the time, it works out, but in some situations, it behaves sporadically, if at all. I'm not saying one is better than the other. I'm simply stating how they work and why Firefox may have issues. Microsoft has historically developed explicitly for IE. Based on this, they make apps that work only in IE. They're getting better, but it'll take time. IIS is one of the tools that falls victim to this. SharePoint, Outlook Web Access, Project Web Access, and a dozen or so others also work only in IE or with severe degradation of capabilities (most simply don't work). Again, sloppy developers are the problem, not Firefox.
You'll notice that most of the developers that work in Microsoft environments use IE exclusively, so they don't take into account other browsers. I'm not saying all, but most - if I had to throw out a number, I'd say 75%, if not more. Without using other browsers, they don't test their code in them. This is where the compatibility breaks down. There are a lot of cool things you can do in IE because of proprietary features, but that just isn't feasible when IE's losing it's hold on the browser mindshare. Maybe not drastically, at this time, but it's happening. IE7 won't change that very much, from what I've seen - it still doesn't compare to other alternatives.
Just for good measure... sloppy developers
