Disable two-finger swipe on Firefox

I like the following solution better (than the existing answer) because it only affects Firefox:

1. In Firefox, go to "about:config"
2. Change the value for "browser.gesture.swipe.left" to "cmd_scrollLeft" (instead of "Browser:BackOrBackDuplicate")
3. Change the value for "browser.gesture.swipe.right" to "cmd_scrollRight" (instead of "Browser:ForwardOrForwardDuplicate")

UPDATE:

There is a native way to do this. Go to System Preferences / Trackpad / More Gestures and deactivate the checkbox for "Swipe between pages" (first checkbox). This works at least for Mountain Lion.

The old, non-functional answer was:

I do not know of a native option in OSX (ML, 10.8.x), but there is a nice trick to do it:

Download the free BetterTouchTool. It lets you map a huge amount of functions to trackpad & mouse gestures as well as keyboard shortcuts. You can set "No action" and you can decide, which applications should know your new custom gesture/shortcut.

So I just mapped the two finger swipe (left & right) to "No action" only for Firefox, Chrome, Safari, Opera and Sleipnir.

Hugely useful and it will solve your problem.