Isn't it annoying that apple doesn't set a mouse back button in os x, or even give you the option?! Well there is a cure for those of us that cannot live without a back and forward button when browsing with safari and firefox.

Unfortunately DCE hasn't discovered a way to script this yet without relying on software, but we are working on it. Though we have figured out how to remap on Windows XP here, figures. Right now we are using a program called Steer mouse. You have the options to configure each mouse button to whatever you like, including a combination of shortcut keys. In this case we want to bind our back button (button 4) to cmd + [ open safari and your backing with ease. You can set the forward button assigning cmd + ] for mouse button 5. My forward button is set to expose shortcut keys (as seen in the image). To do this you must set temporailty unbind your expose setting to have the key register in steermouse.

Download the steermouse installer here