Private browsing mode has been included in the latest nightly release for Firefox 3.1 beta. This is a feature which didn’t quite make it into Firefox 3.
Private browsing allows you to browse the web “anonymously”. No cookies, cache, history, temporary files, or saved passwords will be stored while in private mode.
To turn on private browsing, go to Tools –> Private Browsing. You can download the latest nightly Firefox 3.1 beta build here.