That's an odd dialog title.

Does it matter what printer is configured as the default in Firefox? This is controlled by the print_printer preference. (If that preference is blank, presumably the system default printer will be used.)

We tried a different printer and got the same result, but still only on this one PC.

I guess that is the current system dialog in Windows 11? Is the print.prefer_system_dialog preference enabled on the aberrant browser? Or maybe I should ask whether it has its default value of false on the working browsers, since I think that is where the dialog comes from.

I'm not sure how the silent and system preferences are supposed to interact.

All of the PCs are Windows 11 machines. All of them have print.prefer_system_dialog as the default of False, including the one that has the issue.

It's hard to understand why you are getting the system print dialog if that is disabled. I wonder whether Firefox is sending some invalid parameters which trigger the display of that dialog. If you disable silent printing, can Firefox print to the intended printer through its own internal print overlay?