First of all put a list of selected urls (one per line) in a file called urls.txt and save it in ~/.mozilla/firefox/*.default/urls.txt. Then put the follow code at the very begin of /usr/lib/firefox/firefox.sh (just below of the "shabang" #!/bin/sh):
ddrs=$(shuf-n1~/.mozilla/firefox/*.default/urls.txt) nwli="user_pref(\"browser.startup.homepage\","$ddrs");" sed-i"/browser.startup.homepage/c\\$nwli"~/.mozilla/firefox/*.default/prefs.js