Opera has issues with Windows live sites,microsoft sites.
Firefox compares better,but has similar resource usage as IE7.So it balances out.Because u cannot go an update from microsoft sites if u are using firefox and many sites also dont open in it.
well, some websites are best viewed with specific browsers ...say now a days most websites are prefering firefox...to IE....but every website is built keeping in view of IE explorer.so IE is universal but when we consider many paratmeters it is FIRE FOX.