TorButton
Description:
This extensions can be used to switch quickly between normal browsing and browsing via the Tor network.
For this extension to work properly, your computer must already have an appropriate configuration: Tor must be accessible on your computer through the TCP port 8123. This is usually done by installing both a Tor client and a web proxy on the computer, with the web proxy, accessible through port 8123, configured to use Tor. In addition, your default configuration to access the web should be direct (i.e. no proxy).
Sources: