I have this problem too. The print I last selected doesn't stay changed either.

Here is what I've noticed:

1) If I change the printer after a pop-up screen brings up the print feature, the printer is not changed.

2) If I go to print a page (say the Google home page) via File Print and change the printer, the printer and settings are saved once I print a page.

This leads me to conclude that changing printer settings on a popup automated print screen does not save.

Open a new tab or page and type in the url bar "about:config" (do not type the quotes - they are there to indicate what you should type)

A warning message will appear because this is a very important file you are editing. PLEASE BE CAREFUL.

Then in the search bar type "print"

Look down the list and find "print_printer". Double-click that entry.

Change the highlighted field to the name of the printer you desire. Click "OK" and then close the "about:config" tab. Now your printer will be the default. Try it now.

NOTE: This works best if you enter the name of the printer as exactly as you can. The name of the printer is what you see in the choices in the pull down menu when you choose a printer in the print dialog box.

Good Luck!