Thought I'd follow up this one, since it ranks top when googling "firefox osx print margins".
Finally got it to work.
What I did;
type in "about:config" in your address bar (without the quotes of course) and hit enter.
in the "Filter" field, type "print".
Now change these values;
Now printing to PDF won't add those pesky margins.
To make it work with your printer, follow the advice above about creating your own custom paper preset.
Hope this helps!