Question regarding how 2 hosts transfer files if they are both behind a firewall

I have looked through the Gnutella documentation, but I cannot seem to
find the answer to this.

The documentation says that if the servent that contains the file to
be transferred is behind a firewall, it can PUSH the file to its
counterparty (who is receiving the file) via GIV in HTTP.

However, what happens when both hosts are behind firewalls? How does
the file transfer happen in this case?