Basically does exactly what I'm looking for, which is a quick switcher for use when working remotely and alternating between work VPN and direct connection. A colour change of the icon shows concisely what connection method you are using.

Not sure why the "PAC Script" option is unavailable, I thought the addon was just bringing the proxy settings from the standard firefox 'options' page up to the toolbar. I need to change that script sometimes and toggle between it and other proxy options. Likewise the 'Auto-detect' tab is greyed out, I might need that sometimes as well.

The settings you make here are not reflected in the Firefox own settings. In fact Firefox seems to override the addon settings, you need to set Firefox own options to 'No Proxy'. WYSIWYG would be nice (reflect addon settings in firefox settings).

The error counter is useful but could become distracting/annoying, fortunately there is an option to toggle it on/off separately at the bottom of the about:addons "options" page for the plugin (linked from 'Options' on the addon's UI panel)

The FAQ page and other links are particularly helpful. I'd like to know more about how the proxy 'auto-detect' works, some additional write-up about this on the FAQ page would be interesting. Also I don't understand why the error log is full of NS_ERROR_NET_ON_CONNECTING_TO errors for files that can clearly be loaded ok?

Very promising - version 0.1.9 / Firefox 59.0.2 / Windows 10 and MacOSX