Got it figured out... comconsole is likely sending the pfSense boot menu and other status messages to the serial port... when I disable the serial port in my system's BIOS, the messages all appear on the monitor. Not a bad idea, I suppose, if I actually had a serial port to connect to! The motherboard has a header for one, but didn't include a ribbon cable or anything for it.
And this is what I get moving my config from a serial console box to a VGA console box... duh. :) In the Advanced settings, the serial console was selected as the default. Sorry 'bout my stupidity on this one!