Press Ctrl + Tab (or Shift + Ctrl + Tab) and you are presented with thumbnails and titles of your opened tabs with the most recently visited ones first so you can more easily  determine which tab you want to go. Hold Ctrl pressed and keep pressing Tab to see the thumbnails smoothly scroll to the left while the status bar displays the tab web address.

Ctrl-Tab, a Firefox extension developed by Dão Gotwald, that has served as a prototype for this change is available from Mozilla Add-ons. The extension provides even more features such as an all tabs preview with search tabs capabilities.