fireproxy од Giulio Pizzini

By clicking on the extension button on the toolbar you switch between these two states by modifying the browser's proxy settings:

1) no proxy

2) socks5 proxy on localhost port 1080 with proxy for DNS enabled

For the plugin to work you need to set 'Run in Private Windows' to 'Allow' in the Option page (right click on the extension button and choose 'Manage Extension').
Also, you need to have a socks proxy active, the extension is only switching the browser proxy settings, not providing the proxy itself