I was able to switch between tab groups on Firefox 4 with ctrl-`, but not on Firefox 5; why?

I updated yesterday from the most recent version of Firefox 4 to Firefox 5 on Mac OS 10.6.7. On Firefox 4 I was able to switch between tab groups using ctrl-` (I loved this feature—I find the mouse-based interface clunky). Now that shortcut either does nothing or shows all tabs at once, as if they were in the same group (although, if I use cmnd-shift-E, the panorama view still shows several groups and if I choose one from there, only one group's tabs display).