Earlier quoted context omitted.
I think it's a function of the endless tussle between browser-as-an-OS and the OS they run on. For example, I absolutely hate that there's no option in Firefox to only use the OS print dialog. Instead I am force to click print, then click "use system print dialog" every time. I seems to remember reading a bug report where Firefox deemed this intentional and wontfix. After all, if you leave Firefox to print something,…
print.prefer_system_dialog = true in about:config does what you want, i think.
Edit: yes it does - perfect!