Normally on Mac,  three finger swipe up will bring the top of the page in Firefox 4 and similarly three finger swipe down will takes you to the bottom of the page, but these gestures changed after Firefox 4 Beta 7 release which now enters you into Panorama.

To make Firefox 4 to bring top/bottom of the page with three finger Swipe up and down gestures,

1. Type about:config in the address bar and continue by clicking “I’ll be careful, I promise!”.

2. Now Set browser.gesture.swipe.down command to cmd_scrollBottom

and set browser.gesture.swipe.up command to cmd_scrollTop

You can also install restartless swipe Top add-on to scroll to top of the page.