Print margins on Firefox (also with Print Edit)?

Question

How can I edit the print margins in Firefox in Ubuntu 14.04? I would like to print a page in A3, with custom page margins, to a PDF...

There are suggestions out there like "setting print margins is done in the printer settings" - but, I absolutely cannot find anything related to print page margins... Here's what I'm trying to print:

In the Print dialog/General, there's nothing related to margins:

In the Print dialog/Page Setup, there's nothing related to margins:

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EDIT: As per comment, the contents of "Paper size" ("US Letter"):

In the Print dialog/Options, there's nothing related to margins:

I even installed the Print Edit extension, also that has nothing obvious related to page margins:

So, how can I control page margins in Firefox in Ubuntu?

EDIT: I found Where is the Print Setup setting in Firefox so top and bottom margins print on the page (now too high and too low)? | Firefox Support Forum | Mozilla Support (2015):

This is hidden and a little obscure... Firefox stores properties for each of your printers, including the "unwriteable" margin distance and the "edge" distance. ...
(1) In a new tab, type or paste about:config in the address bar and press Enter. ...
(3) Double-click the print.printer_BrandX.print_edge_top preference and enter your desired distance in hundreds of an inch. ...

So to update the question - is there a way to set print page margins in Firefox from a GUI (without changing about:config settings directly)?

Answer 1

Click on the Main menu and choose Print

Go to the top and clik Page setup

Then click on where you would choose A3 and go to the bottom of the list to Manage custom... click on that option and you'll be presented with this box.

If you want to use Chromium in the future just press CTRL + P and click on Margins -> Custom on the left panel.