An unfair comparison:
PID USERNAME THR PRI NICE SIZE RES STATE TIME CPU COMMAND
107 ptribble 4 49 0 1024M 971M sleep 219:15 1.75% firefox-bin
25551 ptribble 1 49 0 64M 52M sleep 0:07 0.02% opera
I still prefer firefox, but that might change. Don't know why, it just seems to work for me better than other browsers. One thing that opera does have that's useful is the ability to duplicate a tab. And I am getting rather fed up of having to restart firefox every day or two to keep memory utilization within bounds on my workstation (or cpu utilization down on the XP box at home).