Torbutton is a Firefox extension that allows you to securely toggle your usage of the Tor Network on and off via a toolbar button. It actually does quite a bit to isolate browser state between toggle modes, and has a good deal of general privacy preserving features as well. Everything it does is described in this design document, including a list of unfixed Firefox bugs that make its job more difficult.

Torbutton is a Firefox extension that allows you to securely toggle your usage of the Tor Network on and off via a toolbar button. It provides a number of features to prevent IP address leaks and correlation of Tor and Non-Tor activity, including cookie isolation, history protection, cache management, timezone spoofing, useragent spoofing, and much more.