A-ha! I’ve been trying to figure out the keyboard command in Firefox to switch between tabs for ages now. The answer is CTRL-Tab. Of course, this means my poor brain will have to learn to switch between that and the Safari command of CMD-SHIFT-[ and -].
Imported comments:
FYI: in case you find yourself stuck on a Windows machine, Ctrl-Tab will switch between subwindows/panes within most Windows applications.