Control-Tab: A New Feature for Firefox from Mozilla was announced a couple of weeks ago. It will work like Alt-Tab (or Cmd-Tab on Mac) works with application switching, except it will switch between Tabs in Firefox. Wish I’d known about this when I wrote How to Work with Tabs in Firefox. Here’s the Ctrl-Tab download page.
Control-Tab without this add-on opens the next tab. If you use that feature of Firefox regularly this would represent a change for you, so don’t download it if you like your current routine.