If you download this program http://blog.boastr.net/?page_id=1722 (better touch tool), you can make your own swipe gestures.

However, I have been able to three-finger swipe forward and back on webpages by going to system preferences > trackpad settings > more gestures and checking the box at "swipe between pages" and selecting swipe left/right with 3 fingers

Thanks, Drew. Thing is that I'm using Lion only on my iMac right now, and, therefore, only the Apple wireless mouse. 3-finger swipes aren't an option for the mouse. When I get around to upgrading the Macbook Pro, then I'll see what options are available, as it'll be trackpad use then.

Forgot to mention that, in the meanwhile, I discovered in Sys Prefs of Lion that I can choose to go with 2-finger swipes for changing pages, but I give up using swipes between open applications. That's okay with me because I've been using keyboard commands to switch apps, and prefer to swipe for page changes.

you could use better touch tool and make 2-finger swipe right/left gestures into shortcuts for: ⌘→; ⌘← respectively

All I can add is that Mozilla & Firefox are aware there are some issues with the release of Lion. Maybe watch other threads for news.

I am not a Mac user but maybe changing settings could help, but you may then loose some other feature. There are some technical reports linked from a contributors thread /forums/contributors/707160

I solved this issue by using those settings:

Trackpad -> More Gestures Swipe between pages = Swipe left or write with three fingers Swipe between full-screen apps = Swipe left or right with four fingers

Yes, that’s fine when using a trackpad. However, on my Apple wireless mouse, 3 finger swipes are not an option. I’ve solved it by choosing to swipe back and forth between pages rather than Spaces. I’m happy to go thru applications via keyboard commands, as I use that less frequently than moving between pages.

I was baffled by the loss of swipe forward/backward too, but try a FOUR (4) finger swipe, it works for me. I don't even see it listed in the trackpad options, (BTW, I LOVE the trackpad for the iMac), but it sure works!

I got flashmxpro's method to work as well. And I actually like it better because I use two finger for vertical scrolling and no longer accidentally navigate back/forward when trying to scroll down a page. This setting will work across other browsers (chrome, safari) as well...

I have been the most loyal Firefox supporter, telling everybody how great it is and making my family install it and all. Now that I got a Mac, I am realizing I have to switch to Safari, mainly because of this reason. It is very sad. Even with the Better Touch Tool it doesn't swipe as nicely in Firefox as in Safari (although I still recommend this app). Maybe I can switch back if this is fixed soon...

This simple fix worked for me, hopefully it works for you too:

System Preferences --> Trackpad

"More Gestures" Tab

Change "Swipe between pages" to "Swipe with two or three fingers"

Change "Swipe between full-screen apps" to "swipe left or right with four fingers"

This way, it appears that you have your two-finger swipes in Safari, and your three-finger swipes back in Firefox.