The new Ctrl + Tab behavior and look was originally planned for Firefox 3 but was put on hold due to time constraints. The new visual tab feature replaces the current tab switching shortcut, Ctrl + Tab / Ctrl + Shift + Tab. And the older tab switching feature is now accessible from Ctrl + Page Up / Ctrl + Page Down.
Although, I expect the tab feature to be like the Windows window switching feature, ( Alt + Tab ), but it doesn’t work as expected so far. The tabs are switched from the current to the last viewed tab, which in effect changes the feature from the old one. A lot of people are already unhappy with this, and want the tab switching from the left to right order as it was in the former ‘non-visual’ feature. There’s a good chance that Mozilla might change as it’s still being worked on.

A little background on this feature has been given by MozillaLinks:

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.