If you have Firefox 3.5 or higher and you are using a new Mac that supports gestures, you can configure Firefox to recognize gestures that let you move between your open tabs.

To set configure this, type about:config in the address bar. If you are given a warning, ignore it.

Once you have the big list of settings, scroll down to the one called browser.gesture.twist.left. Double click on it and then enter Browser:PrevTab into the text box.

Next find the setting called browser.gesture.twist.right, double click on it and then enter Browser:NextTab.

Once that's done, you'll be able to use the twist gesture to switch between tabs in Firefox; twist left to go to the previous tab, twist right to go to the next tab.