I know this isn't a direct and particularly useful answer, but I've recently setup a VPN with both a McAfee/Secure Computing Sidewinder, and with a Juniper SSG ScreenOS device, and on the PAN side of things I simply followed their guide and to my surprise (as someone who hasn't played with VPN's much) it pretty much worked fine first time - so unless the Firebrick does something weird I don't see you should have any problem.
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