The extension use its own settting independent with proxy settings in firefox preferences. The proxy setting in firefox preferences should be set to "direct" (no proxy) for this extension to work (at least on my FF59). This is not mentioned in the introduction or some where else. Anyway finally it works.

In the past there are addons whcih direcly toggle firefox proxy settings, e.g. QuickJava (http://quickjavaplugin.blogspot.fr/p/welcome.html). Now with the new FF57 extension system I don't koown where to find this kind of extensions.