I need to switch to anonymous surfing mode for specific sites. Is there a Firefox addon for anonymous web surfing?
Is Tor (The Onion Router) network something for you? With Tor, you are anonymous for everybody in the network, but it requires to install software on a PC. There some warnings in the FAQ page about the usage of Tor.
Torbutton is an extension that will switch the Tor usage for you.
If you just need the browser not to record usage data (cookies, history, cache), then you can use the Privacy Mode in Firefox.
There's also an extension called QuickProxy that allows you to switch the Proxy status fast. You can use an anonymous proxy somewhere abroad to hide your visits.
I use the FoxyProxy add-on which does a great job of switching proxy servers for me. They have an additional, subscription-based proxy service, which I've never used, but seems reasonably cheap ($20 for 3 months).