Ok, sorry to keep poking at this issue, but I think I was too hasty in blaming firefox.

As I previously mentioned, IDLE "forgot" that Ctrl-N meant "next line" and reverted it to "new window".  I fixed that behavior by changing and resetting the key set, HOWEVER, now the multiple-key sequences like Ctrl-X-Ctrl-S are broken again.  Closing all other programs, rebooting the system, defaulting my IDLE configuration -- none of that makes a difference.  Truly I'm baffled.

Is there actually code in IDLE/Tkinter to handle those kind of keyboard shortcuts, or is that a Tcl/Tk feature that is merely exposed by the Python side?  If the former, could someone point me in the direction of said code?