The ONLY problem with internet explorer is activex - such a great technology with such a stupid implementation, and thus such a great probability of misusing it.If you simply disallow activex on your internet explorer, you'll practically get to 80% of security that firefox and opera have. With activex enabled, that level is below 20%...
I am kinda used to use firefox - it is a natural companion to my browsing habits since netscape navigator first appeared.
Positive sides of firefox are strict compliance with w3c standards and html/css coding standards, which gives you perfect web page, if you follow same standards. Internet explorer uses different engine, which has its own standards, which are almost-compliant, but not 100%. So if you ever stumble upon a site that is shown properly in internet explorer, and not in opera or firefox, that has to do with moron web designer, not with bad browser...