I would have thought that you would want to have the firewall with one interface into each of the 2 VLANs, and no possibility of each VLAN communicating with the other EXCEPT via the firewall.
Whether the procurve switch is secure enough for this purpose (and you are able to configure it correctly) is not something that I can answer - I'm sure that you can get information from the HP website.
Having implemented secure networks for corporate, Government and military clients, I would not dream of putting the external and internal networks on the same switch.
However, the risk may be acceptable in a home environment.
SD