Till now keyboard shortcut for Firefox Panorama aka Tab Candy is Ctrl +Space is for Windows and option+space on Mac and now it has been changed and newly assigned one is Ctrl +E for Windows and cmd+E on Mac.
Old shortcut still works for Users using previous versions of Firefox 4 beta, however new shortcut will activate Panorama in latest Firefox 4 nightly builds and in upcoming Firefox 4 beta 7.
Firefox Panorama keyboard shortcut
Ctrl +E for Windows
Cmd +E on Mac.
Update : Mozilla changed Panorama shortcut yet again its Ctrl+Shift+E now,