I switched from Firefox to Opera a few months ago. There was too much code bloat on Firefox. It’d run with 400MB+ processes when I had several tabs opened. Opera isn’t a processor hog and runs much faster. Its disk caching operation is much better. It also has more features and options imo.
Opera was my standard browser from about v2.xx, but they hosed the UI in whatever version came after 7.23, and I couldn't use it anymore. As configurable as it is and as hideous as the change was, I was surprised someone didn't do a plug-in to simulate the 7.23 UI (or a built-in option). Now I use Firefox.