In Firefox 4 there appears a small arrow at the end of the tabs bar entitled ‘List All Tabs’. Clicking on it drops down a menu that lists all opened tabs. This is a handy feature intended to make navigation easier when you have too many tabs open. Although made visible in Firefox 4, this feature was introduced in version 3.6 but hidden by default. There is another hidden feature - two actually - that adds thumbnail previews to this list as well as Windows like Alt+Tab switching.
To enable thumbnails in List All Tabs, type about:config in the address bar and search for the parameter browser.allTabs.previews. Double-click the entry to set the value to True.
You will notice the ‘List All Tabs’ arrow has changed to a grid like icon.
Click on the icon and you will see rows of tabs with thumbnails instead of the list.
To enable Windows Alt+Tab like switching, type about:config in the address bar again and search for the parameter browser.ctrlTab.previews. Double-click the entry to set the value to True.
You can now press Ctrl+Tab to cycle through opened tabs. Like Windows Alt+Tab, you will be able to have a graphical preview of each tab.
[via Firefox Extension Guru]