I am an Opera user myself, but I don't find either one to be better all around. It all depends on what you're using it for. Opera's good for me as I like a fast browser without much bloat and I tend to like better html, and Opera can be anal about that
It's surprisingly compatible in some cases >_>
To some extent I find it to be between Firefox and IE, in that it doesn't take five minutes to start, and loads pages faster, and it scrolls differently.
Firefox is more comfortable for some users, and it is less anal about html, and isn't internet explorer >_> Firefox does have its other good parts but everyone's heard those too many times.
I've used Opera, Firefox and, obviously, internet explorer. I have to say Firefox seems to be the best for general users - I found Opera to be a little confusing, bulky and limited.
Opera's no more limited than Firefox... And I don't see where the confusion stems from. You have the toolbar, then tabs, (you know tabs, remember firefox?) then a bunch of buttons with obvious uses, which you've seen before. There are two added buttons, rewind and fast foward, but they're unimportant. The address bar is below that and then a google search bar. What's so confusing? It's almost a copy of Firefox and Internet Explorer... o_O
As for bulky... Look at the memory usage for firefox, and compare it to opera. You're calling OPERA bulky? Opera definitely runs smoother and loads faster, and it seems to take up much less ram and processor. Firefox is quite a memory !@#$% with multiple tabs open. It keeps five pages or so (I've forgotten exactly) for each and every tab preloaded in ram. This is not good if you use tabs heavily
It's not that bad if you're a normal user, but I keep wayy too many tabs open on average, as that's how I work best.
As I said, it all depends what you want from the browser.