Mozilla officially started supported Tracking Protection in Private Browsing mode with Firefox 42, which launched a couple of weeks ago. Congratulations to everyone who worked on the launch! The onboarding looks awesome and the unified UI is a nice touch, although I have to admit a preference for the original, engineer-designed marketing aesthetics pictured below.
Even outside of Private Browsing mode, you can still take advantage of Tracking Protection by going to about:config and turning on privacy.trackingprotection.enabled. This behavior has been supported for over a year since Firefox 34, so it's great to see Mozilla making this more usable by turning it on in Private Browsing mode.
I hope that Mozilla continues to use its products to challenge the notion that we owe our eyeballs, our computing resources and our entire browsing history to the ad industry, with no questions asked.