In the early days of development of Firefox 3.1/3.5 there was a planned ‘Tab Preview’ feature for displaying thumbnails when switching between tabs similar to the Windows Vista/7 Alt+Tab interface. For whatever reason this feature ended up being scrapped from the final Firefox 3.5 release in June 2009.

This feature has been included in Firefox 3.6, but most users don’t know it is even there. Good reason too, it is disabled by default. To enable the Tab Preview feature:

1. In a new tab type about:config in the address bar
2. Filter for browser.ctrlTab.previews
3. Double-click the entry to set the value to True

Once this feature is enabled, the Display All Tabs button can be set to display the ‘Tab Preview Panel’ (shows thumbnails) of all the open tabs:

1. In a new tab type about:config in the address bar
2. Filter for browser.allTabs.previews
3. Double-click the entry to set the value to True

Source: Mozilla Links