Guess what! You can use multi-touch to switch tabs in Firefox 3.5!!!
It was enabled in the beta's, but it's disabled by default now that 3.5 has gone gold, but here is how to enable it:

• Open a new tab (go on, do it now)
• Put “about:config” in the address bar, no quotes.
• Beware the dragons. Are you’re sure? Yes, you are. You are okay.
• Now type “twist” into the search box.
• Double-click on the twist right, and put “Browser:NextTab” into the box, without the quotes.
• Then, in twist left, put “Browser:PrevTab” without the quotes.
• You’re done!
• It applies immediately, try putting one finger down and moving the other up or down, cool huh?
• I changed one other setting, the threshold one, from its default (25) to 10, to make switching faster.

You can also use the following multi-touch operations:

• 3 fingered swipe down or up to jump to bottom and top of the page
• 3 fingered swipe left and right for back and forward
• Pinch open & close to alter zoom level of the page

Let me know if you find any more.