To create a new set of default bookmarks, at best configure a fresh firefox profile so that the bookmarks match your needs and
export them in html format. Use the Bookmarks->"Organize Bookmarks..." dialog to do that.

To make firefox use a custom bookmarks.html, you need to set a pref. The example below assumes you ship your bookmarks.html in a custom-data package.
{{{
pref("browser.bookmarks.file", "/usr/share/custom-data/bookmarks.html");
}}}

Put that pref in a custom-bookmarks.js file and deploy it to one of the following locations
 1. /etc/firefox-3.X/pref/custom-bookmarks.js
 1. /usr/share/ubufox/defaults/preferences/custom-bookmarks.js

Downstreams can either ship the /etc/ file in a custom-data package or - if they do this anyway, maintain a fork of ubufox that ships the custom-bookmarks.js file.

'''Remember:''' bookmarks.html file is imported during profile creation. To test, move your ~/.mozilla folder away and start firefox.