I hate Opera. I didnt even want that crud on my computer, but somehow, it installed itself, and it took FOREVER to remove. I had to search the forums online. It was the unwelcomed guest on my computer that refused to leave.
I like and have used both extensively. I'm using Opera right now though being that it's more feature-packed right out-of-the-box, and I don't have to install a bunch of extensions to get the functionality I want. Opera also uses less RAM.
Firefox wins in regards to customizability, though.