The Invisible Internet Project(I2p) is described by its makers as
an anonymous overlay network – a network within a network. It is intended to protect communication from dragnet surveillance and monitoring by third parties such as ISPs.
The video below is is a great resource on how to configure FoxyProxy addons to work with I2p:
To make it work, make sure of the following:
1) Make sure FoxyProxy is set to “Use proxies based on their pre-defined patterns and priorities”
2) Use 127.0.0.1 as the proxy hostname and
3) in the URL patterns setup, make sure there’s a period in your wildcard pattern: *.i2p/*