Comment 109 for bug 10910

If you have problem to get A4 as default paper size instead of US Letter:
1) Close firefox!
2) Edit the pref file in your profile (my profile is named tnju3mjg.default)
Eg.
gedit /home/username/.mozilla/firefox/tnju3mjg.default/prefs.js

Search for user_pref("print.print_paper_
Change "na_letter" to "iso_a4" and the height and width as this:

user_pref("print.print_paper_height", "297,00");
user_pref("print.print_paper_name", "iso_a4");
user_pref("print.print_paper_size", 3);
user_pref("print.print_paper_size_type", 1);
user_pref("print.print_paper_size_unit", 1);
user_pref("print.print_paper_width", "210,00");

further down, for your default printer (my printer is Arbetsrum101):
user_pref("print.printer_Arbetsrum101.print_paper_height", "297,00");
user_pref("print.printer_Arbetsrum101.print_paper_name", "iso_a4");
user_pref("print.printer_Arbetsrum101.print_paper_size_type", 1);
user_pref("print.printer_Arbetsrum101.print_paper_size_unit", 1);
user_pref("print.printer_Arbetsrum101.print_paper_width", "210,00");

I did the same for all my printers

3) Save the file
This solved my problem.