Jacques,

While it most likely could be an issue with the printing functionality in Firefox, most of the time applications punt to the system (Windows OS, Mac OS X, Linux) printing functionality and basically send the page to the print spooler. However, there's quite a bit that can be customized beforehand within the application (Firefox). I'd first check out your printer by looking at its Properties (right-click on the printer icon) in Windows (or whatever OS) and setting your "Printing Preferences" to double-sided, flipped. This not only insures you get the right type of printing with Firefox, but also your other applications as well. I have this same functionality with my HP 8600 Officejet Pro and have my duplex printing preferences (to save on paper) setup like I've described here to you.

If you're getting the new printing interface (which looks like this) then the ability to choose which side to flip along hasn't been implemented yet. That's tracked in bug 1671702. You should be able to click on the "Print using system dialog" link there though and get to the old interface which should let you choose between long/short edge as per the screenshot Joe provided above.