Firefox is way much better.
Those are the reasons why firefox is better,
Firefox has an integrated inline spell checker
Opera doesn't have an integrated spell checker nor
inline spell checker
In firefox you can customize it the way you want and you can download extentions as many as you want.
Opera doesn't have those fearures
Most, if not all, the websites today are compatible with Firefox
Many websites still don't support Opera
When you use the find option in firefox, you don't need to write to full word to search it. Firefox will search the word that you are looking at while you are typing it. For example, if you type "F" firefox will search for a word that beggins with F, then if you you type "Fi" then firefox will highlight a word that has Fi.
In Opera, you need to type the full word in order to find it and that would waste a lot of you time if you are doing some research.
Finally, Firefox looks more sexier and better than Opera