Do you know much like Firefox 3.6 Firefox 4.0 also shows thumbnail previews of tabs when we use Ctrl +Tab keys combination but this feature was disabled by default in Firefox 4.0 we can enable it easily.
Enable Ctrl +Tab previews in Firefox 4.0
1.Open Firefox 4.0 beta 1 browser, and type about:config in the address bar and click I’ll be careful, I promise
2.Now type previews in filter and double click browser.ctrlTab.previews to set to true.
Now you can able to switch between thumbnail previews of opened tab using Ctrl+Tab in Firefox 4.0
This feature is also available on FireFox 3.6 and enabling it by doing how you do above. Not sure when it was first implemented.