As stated above, due to the move by Mozilla to Webextensions they have removed access to about:config which holds various settings for Firefox. My add-on and other add-ons that change proxies via this method have become unusable with this move. Please don't blame me, this was beyond my control and as you move to Firefox 57+ my add-on will become unusable :-(
Toggle Proxy enables you to toggle between two proxies by left-clicking on the icon in the status-bar (or via the shortcut key ('alt + x' version 1.3)) or the toolbar icon (added in version 1.7 for Firefox), useful if you use different proxy settings at work and home. (e.g. auto=home, manual=university). The two proxies to toggle between can be selected via the preferences, availble via the add-on menu in the tools menu or right-clicking on the icon in the status bar.
Versrion 1.7 now includes a toolbar button for Firefox.
Version 1.5 now includes support for Thunderbird.