Firefox used to be my main browser, then it started to kill my internet, and to browse I had to reinstall IE, and I knew I had to get off it, so I got Opera. I'd say Firefox performance and security, and Opera as Feature rich and has good HTML interpreter (stricter than Firefox or IE). I use both Firefox and Opera, never leavin emm behind again (not that I ever did). Oh yes magically Firefox started to work again, can't say the same for IE
