Of course, there is the CSS question and the JScript one where Microsoft implemented it's own instead of using industry standards, but that's not what I was refering to. Just for straight HTML, things won't display the same sometimes in IE and other browsers because IE lets you do mistakes while other browsers don't. You can forget the close a tag in Explorer. You can forget a TD in a table. You can write lazy and sloppy code and IE will still work, while other browsers won't.
And you're wrong. If I have a client using Firefox for example and his web site looks like crap in it, he won't blame Firefox, he will blame me (the web developper). If anybody using Firefox goes to a web site and the site doesn't work right, they won't blame Firefox, they will blame the site developpers. I do, most Forefox users I know do too. Because they IS a way to make it look right for both. Unless you're relying on Active X of course, but I've never liked the idea of Active X anyway.
As far as IE integration goes. Type C:\ in the address bar of Explorer. Now do the same in Firefox or any other browser. 'Nuff said.