Instead of using toolbar buttons, you can employ Opera's keyboard shortcuts.
To go to the previous message in Opera:
- Press U (just 'u').
To open the next message in Opera:
- Press J.
Instead of trying to remember them with mnemonics, look at their arrangement on your keyboard. You can reach them both with your right index finger. If you don't move your index but just press, you go to the next message. If you move your index up one notch and then press, you go to the previous message.

