Firefox 3.1 is definitely one of the fastest versions made by Mozilla, in addition to that it has several new features including the addition of private pornmode browsing where users can browse websites without saving the history and files to the cache.
By default the private mode browsing can be enabled from Tools > Private browsing, however there is a extension that will help you toggle between private browsing modes in Firefox with a single click.
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Using the extension you can easily auto-start Firefox in private browsing mode and switch back to regular browsing mode. The extension is experimental and will only work on Firefox 3.1 beta 2 versions of Firefox.