"status bar/toolbar icon to toggle between two proxy settings which can be set in the preferences or toggle via a shortcut key (alt + x)"
"Transparent proxy server that works as a poor man's VPN. Forwards over ssh. Doesn't require admin. Works with Linux and MacOS. Supports DNS tunneling."
Statusbar button to turn the proxy on and off with a single click. Outdated but reported to still work as of 2015. More up-to-date alternatives are https://addons.mozilla.org/en-US/firefox/addon/toggle-proxy-51740/ and https://addons.mozilla.org/en-us/firefox/addon/quickjava/