Switching to WebExtension some features of the MM3-ProxySwitch are missing. Many users asked for it, including the feature "Automatic switching" (testUrl).
This feature is now implemented. This requires the possibility to check a URL for accessibility. This is implemented via a "HEAD" request to the server.
We documented this in two sections: New: https://Proxy-Offline-Browser.com/ProxySwitch/News.html#2018.332
