As far as I understand, the default bookmarks are hidden in /usr/lib/firefox-n.n/omni.jar, you can modify this file to change default bookmarks for all users. Anyway this is not a good practise and I hope providing defaults will get easier in time.
A nicer way is to provide something in a default profile directory in order to let migration assistant do importing the bookmarks, when new Firefox profile is created. I think you have to provide profiles.ini in /etc/skel/.mozilla/firefox.
And under /etc/skel/.mozilla/firefox/5ge4y5n2.default you can provide your files to be imported to a new profile, for example bookmarks.html or places.sqlite.
Only problem with this practise is that it allows only one Firefox profile per user and deleting the profile and creating new one loses the defaults.