I don't understand why, on my laptop, Firefox (aka Ice Weasel) has a full list of the printers listed in /etc/printcap and Thunderbird (aka Ice Dove) doesn't. Thunderbird only lists Postscript/default. I thought both these applications, being gnome applications, got their information from the same source.

In Thunderbird, I went to Preferences > Config Editor and changed the value of print.printer_PostScript/default.print_command to lpr -Plp1.

I still don't have the list of printers that I'd like, but at least Thunderbird is printing to the printer I currently want.