I know this is an old thread, but in case anyone else is looking for information on the topic I will add what I have experienced.
I ran pfSense in a VirtualBox VM for a couple of years with no problems and then it quit working. What I discovered recently was that two problems coincided. First, I had set up the host using bridging from bridg-utils and then used bridged adapters from the guest. This was overly complicated and I confess that I never fully understood the configuration that worked.
The second problem was that my ISP changed (quietly without notice) to limit my connection to only one IP at a time and set that IP on a different subnet than the DHCP server. This caused the host to grab the IP address on boot and then pfSense in the guest could not get one.
I believe globexgr is correct in the recommended configuration. That would be to set the host to grab the IP from the ISP and then do a "host-only" connection from the guest VM red interface to that host net.
Then I would do a bridged output from the Guest to a static IP on the host Green interface. Alternately the host "green" adapter could be set with another IP net address and the guest could host-only into that net.
All in all the best advice I have seen is Draw The Network Diagram including the virtual parts. That is what I intend to do before I dive back into the current firewall and see if the new setup solves both problems above.
One caution to all this, however, it is my understanding that if you use a pure bridged connection from Guest to Host then you bypass the Host TCP Stack. If your VM is a firewall that would be safer than using host-only. But everything I have read says that I am not able to do that because a bridged VM adapter cannot work on a "single IP" type connection like my ISP implemented :-(
Hope this is useful to someone.