same thing is happening to me, a lot of sites don't work; facebook doesn't work, macrumors looks like this:

Don't know if it was the same problem but I fixed everything by going to Tools>Clear Recent History>Selecting Time Range: Everything and checking the Cache box.

To use the back/forward on FF, go to the trackpad prefs and the mouse gestures page and switch Swipe between pages to two or three fingers and change Swipe between fullscreen apps to swipe left or right with four fingers.

Thanks. That worked great. Not sure why two finger swipe doesn't work but I can adjust to three fingers. At least I can continue to use Firefox. I was afraid I was going to have to use Safari, not something I was looking forward to.