Might be faster. Only blake will use this, so asking him for input.
Unless mxr is lying to me, JSProxyHandler appears to be gone. Please reopen if that's the case.
It's now js::ProxyHandler. I don't know if the bug is still valid, though.
This actually got fixed as part of the patch for bug 574299 a long time ago (see jsproxy.h, js::ProxyHandler::family is an inline function and not virtual).