I have a Brother printer that supports front and back, from libreoffice and okular it works correctly, from thunderbird even if I set front and back it prints on a single side for page, what could it depend on?
To print front and back with Thunderbird
- Edited
FAb7D https://support.mozilla.org/en-US/questions/1124376
Sounds like something with thunderbird.
Maybe try printing different doc.
Here's something you might try as a workaround. When you go to the print dialog in Thunderbird, scroll all the way to the bottom. You should see a link that says "Print using the system dialog". See if you can print double sided with that.
If you want to solve the problem without the workaround, I'd recommend looking through Thunderbird's issue tracker to see if this has already been reported. It sounds like Thunderbird is not handing off the chosen options properly to the printer driver.
Good luck.
- Edited
As a trick I also had the option of printing to pdf first and then printing the pdf with okular. However, I think I have solved it, although I have to see if it lasts over time 
, I had two drivers: the "driver less" (the one of default printer) and the driver (build from rpm), but I used the "driver less", now I have uninstalled the driver (from rpm) and left only the driver less and everything seems to work better. I understand that's not much of an explanation, but that's what I did.... consider that the brother driver was derived from an rpm and even if I was not using it maybe it created some problems....
Thank you all for the suggestions.