Jeff and John-Michael, thank you both for your responses.
Toggling between Frames/Timecode in General Settings has no effect. I’ve also restarted, and created new sequences, to no effect.
Toggling Show Audio Time Units in the Sequence drop-down switches it between raw frame numbers when it’s off (no min,sec,frames) and complicated numbers (00:06:47.874) when it’s on.
All timelines in all panels are only showing a cumulative frame count, with no division into minutes and seconds. It’s probably the result of me accidentally hitting a wrong keyboard command or something . . .