Seeking help on IRC, the suggestion to change the pop.cue lines to:
pop.cue( 12, function() {layover("One"); });
pop.cue(168, function() {layover("Two"); });
pop.cue(336, function() {layover("Three"); });
pop.cue(506, function() {layover("Four"); });
pop.cue(703, function() {layover("Five"); });
worked.
(Now to see why they work in Chrome but not Firefox...)