Did it use to work with previous versions of Firefox?

Now that you mention it, yes. I don't recall the last time that FF updated to a new version, but I didn't have this problem in the past.

ashergabbay said

The duplex printing is set as default in the printer properties.

Check it from within Firefox, as the system wide settings may not be used or passed thru to Firefox. I have seen that with other Brother and with 2 different Epson printers (with Epson it has specifically said "Not Available" when I was using Firefox, where it worked in any Microsoft program.)

I am working with a Brother MFC-J615W inkjet, but most Brother printers have their various user setting in a similar location in the Brother printer software. You need to do this thru Firefox. 1. {Ctrl+P} then make sure that printer is selected at Printer - Name. 2. Then open Properties. 3. Next select Advanced and then maybe Other print options 4. And if you don't see that preference there, look at all the various menu categories for that pref.

I use Brother printers for two reasons; the ability to use generic ink cartridges and their unmatched support for Linux operating systems. At least when I switched to Brother back in 2008, no one else 'did Linux' as well as Brother.

Yes, of course I checked it from within Firefox, opening the printer properties and making sure they're set to duplex. Except for the standard duplex setting, I went through the entire preferences to see if there was another setting. None.

Could you test with a clean new profile, please. https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

Thank you Oxylatium! That worked.