The feature also blocks some ads, but that's not its primary purpose. If ads don't contain trackers, Firefox 42 will load them.
The new private browsing mode also has a "Control Center" that provides quick access to all of Firefox's security and privacy controls. It's a notable enhancement, because it makes it easier for users to take advantage of the safeguards. And because some websites won't load if you disable tracking, Mozilla added a button to the toolbar that lets you turn off the no-tracking feature for individual sites.
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