How To Set The Default Paper Size To A4 in Firefox

I am running Firefox on Ubuntu and the default paper size was US Letter.  I wanted to set the default paper size to A4.  There isn’t a graphically interface to do this.  So you have to:

1. Visit about:config
2. Search letter
3. You should have a result with a value of na_letter.
4. Double click to modify and change the value to A4

Then it should look like this:

Hopefully that helps someone change the default paper size from US Letter to A4.