In a world that is ever so cruelly dominated by IE, we are fortunate to have with us alternatives. And so we all get busy figuring out which is the best alternative! Not everyone will love Firefox, or for that matter hate IE, and understandably so. Individual preferences vary and so does the usage pattern. So when it comes down to drawing a comparison between these alternatives, it ain't an easy job. All the more difficult, when the alternatives happen to be much superior to the original and similar to each other in ways more than one the way Opera and Firefox are.
Again, these are individual preferences, and the comparisons may go on and on without us ever being able to substantially claim which is better!