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There are also other factors involved with browsing efficiency and speed.
- Opera's fastback cache system is the best in my experience
- Opera has a lot more shortcuts. You can easily add more through mouse/keyboard shortcut configuration which is limitless.
- Opera's UI is more responsive than at least Firefox (XUL overhead. XUL is way more abstract than Opera's themeing engine). IE is a toss-up.
There's more too.