Aggravatingly, the "side" click, what you get if you squeeze the sides of your mouse (if your mouse supports it) is by default mapped to "back" in several browsers. Since squeezing your mouse is what you accidentally and subconsciously do when you pick up the mouse, this is almost never what you actually want. Helpfully, Ubuntu has deeply hidden away any means of configuring this behaviour.
With assistance from the OSM help board I found this post about fixing it, for which I've given an archive link because the current version of the site has taken it away (although search engines still index it....) Anyway, here's the short version:
Section "InputClass" Identifier "Side mouse button remap" MatchProduct "Mitsumi Electric Apple Optical USB Mouse" MatchDevicePath "/dev/input/event*" Option "ButtonMapping" "1 2 3 4 5 6 7 0 0 0 0 0" EndSectionexcept replace the product with your mouse device and in the ButtonMapping make sure you keep the buttons you use and zero out the correct side-click button for your device.
"When we react out of fear, when we change our policy to make our country less open, the terrorists succeed---even if their attacks fail. But when we refuse to be terrorized, when we're indomitable in the face of terror, the terrorists fail---even if their attacks succeed." --Bruce SchneierPosted by blahedo at 9:08pm on 11 Jul 2013