In the "native print dialog box" (what you see with ) select the printer you want to use. Firefox remembers the last printer you used and defaults to the last printer that was used. When you select and use a different printer, Firefox will "default" to that printer the next time you go to print something.

Can I set Firefox such that it always defaults to the Windows Default Printer?

Not that I am aware of.

Thanks for your help. FF's behaviour is different from that of most other applications I use. Perhaps a future version might have defaulting to the System Default printer as an option.