I've been a registered Opera user since v3.5, and always thought it was far more innovative, smaller and faster than all the rest, but with more annoying stability, page rendering and javascript bugs. Opera 8 is definitely a pretty sweet and tightly coded browser, and finally has a nice and relatively uncluttered default interface.
For me, firefox is a bit slower loading, slower rendering and bloated, but more stable and renders more sites more accurately. It can be loaded up with useful extensions too (esp adblock).
On either windows or linux I tend to switch between firefox 1.03 and opera 8 depending on how I feel! IE doesn't get a lookin...