From Firefox 4, the pinch gesture commands that allow zooming in and out of web pages, have been ommitted from the settings. The entries are there but no associated commands.
Here is how to restore the commands and get the pinch gestures working again.
1: Open Firefox
Then in the address field (1) type about:config
2: Search for the Pinch Gestures
Scroll down to find browser.gesture.pinch or ...
3: Filter the List
Enter browser.gesture.pinch into the Filter field (1).
4: Select the item
Double click the line browser.gesture.pinch.in
5: Enter the command
6: Command has been added
You should now see the command appear in the Value column.
7: Repeat steps 4 to 6
Repeat for the following items and their associated commands:
8: Close Firefox and re-open it.
Page zoom should now work with pinch gestures on your trackpad.