As announced before, 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.
While eye candy doesn’t hurt, I am not sure black is the right background color for all platforms. More tab thumbnails may also help find the one I’m looking for faster, as in earlier prototypes.
As said, there’s also a new behavior for Ctrl + Tab. Instead of just moving left to the next opened tab, now it switches between the current and the last viewed tab and I have to say I’m already loving it. I usually have to check several other pages while posting and to ease the tab switching I moved the reference tab next to the post edit tab to have it a couple of keystrokes away. There’s no need now: I just took the long way to the reference tab once and then it’s already at reach.
The new Ctrl + Tab behavior and look was originally planned for Firefox 3 but was put on hold due to time constraints.
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 including an all tabs preview with search tabs capabilities that replaces the tab list menus when pressing the List all tabs button in the tab bar edge, and an option to switch between tabs in all windows. At least the all tabs preview option is also being considered for inclusion in Firefox 3.1.