the reason some websites dont work in Opera and Firefox is because each browser works differently, some obey the
W3 Standards and some *cough* IE *cough* completely ignore them.
Add a non-standard browser with a huge market share, and you get web developers who think "oh it works on IE, it'll be fine" and so their badly coded site which works fine in IE, looks terrible in a browser that is W3 compliant.
It can be annoying if you try to make a compliant site, as it'll often not work in IE, so you have to add in 'hacks' to try to make it look ok for the majority of web users.