Firefox should list the print_duplex value for each printer. Printers you have not yet used in the new print experience will have a weird number that is not on this list:

1. 0 => print single-sided
2. 1 => print double-sided, with long-edge (portrait mode left-right) binding
3. 2 => print double-sided, with short-edge (portrait mode top-bottom) binding

To edit a value, double-click the current value to display an editing field. Change it as you wish, then press Enter or click the blue check mark button to save the change.

Does the correction from 2 to 1 work and does it stick?

I solved the problem myself using Troubleshooting Information