For the longest time I've had issues with the back/forward side buttons on my Intellimouse Explorer mouse in Firefox when I used my Mac laptop. And I've also had trouble with middle-clicking links not opening in new tabs or middle-clicking a tab not closing.

So I searched around and didn't find completely clear instructions. But I've finally been able to make my mouse behave the same on the Mac as in Windows when using Firefox.

Here's the steps I took:

1. Install the Intellimouse drivers for OS X. You can get them at the Microsoft Download Center site. The latest version of the drivers, as of this post, is 6.0.
2. Open up the Microsoft Mouse preferences tab in the System Preferences of OS X and click on the Buttons tab.
3. For "Wheel Button" assign it to "Handled by Mac OS".
4. Check to make sure the thumb buttons are assigned to Forward and Back.

You should be good to go!