Tested in Windows. Unfortunately Virtualbox bridged networking under Linux tends to be broken.
noping noconfig homepage=about:blank
nopingdisables the machine ping for statistics as documented in privacy
noconfigdisables the configuration service
Using bridged networking (it's NAT by default) and filter in Wireshark on the
eth.src address of your guest.
Besides NTP (for accurate time) and the usual router pings and such, you should see no automatic connections. This actually takes a lot of time and effort to turn off Firefox's automatic connections since they are so poorly documented and new ones seem to be constantly introduced.
Related project: https://github.com/pyllyukko/user.js