A dropdown ‘Print’ box appears when I tell Firefox to print. It includes an item ‘Two-sided printing’ with options ‘Off’, ‘Flip on long edge’ and ‘Flip on short edge’. As my item is in portrait format, I have selected ‘Flip on long edge’. This is the same as the default setting on my printer.

Not using any add-ons. I tried a different approach, however, following the link at the bottom of the dialogue box which says 'Print using the system dialog ...'. This took me to the usual print dialogue and, when I told it to print, both pages came out correctly. The next step was to see whether this fixed the problem using the dropdown without going to the system dialog, and it did. So the problem is fixed, but I'm not sure how or why! Thanks for your efforts.