1.5.0.2 Plugins is not wired up properly
It looks like you did not update the .nsi for the pluginsdirectory new location?
PluginsEnvironment:
;=== Set the plugins directory if we have a path
StrCmp $PLUGINSDIRECTORY "" UserProfileEnvironment
IfFileExists "$PLUGINSDIRECTORY\*.*" "" UserProfileEnvironment
System::Call 'Kernel32::SetEnvironmentVariableA(t, t) i("MOZ_PLUGIN_PATH", "$EXEDIR\$PLUGINSDIRECTORY").r0'