If it helps I believe A4 is exactly 2x the area of A5. Or put anther way A4 folded in half is A5.
The letter size in Europe is likely to be A4 whereas prior English, and American Letter size is approximately similar to A4.

You should be able to set Firefox to print as required. See

• How to print web pages in Firefox
• Fix printing problems in Firefox

I know how to set my preferences when I print, but I have to do that every time I print. I want Firefox to save my preference so every time I print it will use that preference without my having to once again set the preference.

Tx, Uncle Dewey

I'm not sure Firefox actually sends those settings to the printer; it may simply use the defaults for your printer regardless of what interventions you try in its settings. That said, you can look at what Firefox has saved from your last print job using the about:config preferences editor.

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the Search box above the list, type or paste printer and pause while the list is filtered

Of the many, many preferences, look for these and see whether they have logical values. If not, you can double-click and try modifying them.

print.printer_name.print_paper_height
print.printer_name.print_paper_width

If that doesn't work (I have my doubts), is it an option to change your printer defaults? What size you do print most often?

A workaround might be to print to PDF from Firefox. There are several free programs for this, including:

• PrimoPDF - http://www.primopdf.com/
• PDFCreator - http://www.pdfforge.org/
• PDF24 Creator - http://en.pdf24.org/creator.html (I haven't tried this one)

In about:config
This is the preference which tells Firefox to save the last settings change.
print. save_print_settings = True - (default): Save the print settings after each print job

This setting has to be changed in about:config if you would want to switch it to False.

And as jscher2000 mentioned -
print.printer_name.print_paper_height
print.printer_name.print_paper_width
- the Values are saved as either Inch or Metric measurements depending upon whether US paper sizes (Letter, Legal, custom, etc) or "A" type paper sizes.

These two settings are saved from the Paper selection made in your printer's Printer Properties dialog window which can be opened from the Properties button in the Firefox "native print dialog" window which is what you see when you initiate printing in Firefox via {Ctrl + P} or one of the Print menu items.

These two settings are automatically set or changed with each print job which is sent to the printer. There is no way to set a permanent default paper size, the default is the last used paper sizes. 

Right, wrong, or indifferent, Firefox does not honor the system-wide default paper or printer settings you make for the printer in your operating system. In Firefox the "last used" is the "default".

Thanks for steering me to about:config. As long as it saves my last document size that will work fine for me as 99% of my documents are letter size. I just wish there were an option to set which tray to use--I have two and I need different tray for Foxfire printing,

Thanks a bunch for your informed reply. Uncle Dewey