Options like this are a part of your printer driver. Other applications, like Thunderbird and Firefox, just hand the document to the printer driver and a printer option window should open. Obviously your printer must have the ability to duplex print. Some printers want you to use the Print command from the File menu to see the printer driver option window. They tend to do just basic printing it you use the Print button on a toolbar.

thankyou for your quick reply. Yes in Thunderbird if I choose 'File', 'Print', rather than 'Print Preview', I do indeed come up with a menu which allows me to choose duplex.

However with Firefox, there is no 'File' button. The only way I can get to 'Print' is by opening 'Open Menu' at the far right of my toolbar (up top), here I see the 'Print' option, if I choose this the web page is shown as 2 pages and there is no option to make it print duplex. Nor for that matter can I see a 'Help' button where I can find the release number of Firefox.

Pressing the alt key or F10 will reveal hidden toolbars. Then go to View-Toolbar and turn the menu bar on.

Thank you so much!!