It looks like this problem related to xulrunner 1.8.1.X relases. In XL 1.8.0, I guess, they had x86 and ppc in one bundle. Since 1.8.1, which is included in JBT 3.1, the probably had them separated. Xulrunner plug-ins for mac stil have filter that enable them for x86 and ppc platforms, but binaries is for x86 only.