Tab switching is getting a dramatic update for Firefox 3.1 in both visual and behavior. Press Ctrl + Tab (or Shift + Ctrl + Tab) and you are presented with tab thumbnails and titles with the most recently visited ones first so you can more easily determine which tab you want to go, have the one you most probably want to go closer, and as a result get to it faster. 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.
If you need or just prefer the old behavior, pressing Ctrl + Page Up and Ctrl + Page Down will fit the bill.
Source:→ Mozilla Links