By default, Ctrl+Tab will switch between open tabs in Firefox. But if you are running Windows 7, you should enable Visual Previews for a sexier way of navigating your open tabs.
1. Open Firefox 3.6.
2. Go to the address bar and input about:config.
3. Click through the warning window.
4. Locate browser.ctrlTab.preview and double-click it so that the value changes to True.
5. Restart Firefox.
Now you have a Visual Preview of what is in each tab when you use Ctrl+Tab.