Just like the Option-3 (or 4) finger swipe brings back back/forward navigation in the Finder history, it also brings back back/forward navigation in Firefox (at least for version 5 -- the one I have).
For those who like the Magic Trackpad 2-finger back/forward navigation swipe in Safari and wish this would be implemented in Firefox, this is the next best thing.
[crarko adds: It's a good bet that over time many applications will incorporate gesture support, beyond the kind of generic type described here.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110807221734791