In Opera, it's right button down, left button click to go back, left button down, right click to go forward. Always.
It's the same in Firefox, until Firefox gets updated and the gestures plugin is broken for a few weeks until the maintainers fix it for the new firefox version.
Going to IE is a nightmare. Then it's right click, click Back on the right-click menu.
Once you get used to mouse gestures, you wonder why anyone would ever waste so much mouse mileage going up to the Back button all the time.