The feature, Private Browsing, would have disabled all caching, cookie downloads, history records, and form data used during the current session. In essence, you could surf the Web and leave no fingerprints.
"It basically said to the browser: I would like what I'm about to do to not be logged anywhere," said Johnathan Nightingale, Mozilla's "human shield," aka its security user interface designer.
He described the private browsing process as this: you hit a button and everything past that point isn't logged. Then, at some point in the future, you hit the button again and it's as though what you just did never happened.
One possible use might be when someone other than the computer owner uses the browser.
"We looked at ways to do this, but the problem is that it touches a lot of code," Nightingale said. "Because there are such rich interactions with Web sites and mashups and things like that, we didn't want to put in something that was half baked."
You can hear more of my interview with Nightingale on my Security Bites podcast here.