Unfortunately it works only within single virtual space (or whatewer it's called in Windows using Win+Tab). It doesn't switch tabs located in different spaces.
Last Tab extension can do that thing:
https://addons.mozilla.org/en/firefox/addon/last-tab/

You can already do this in Firefox, just go to options and turn on "Ctrl+Tab cycles through tabs in recently used order". It also shows tab previews of your most recently used tabs.