To enable Tracking Protection in Firefox 35 and later,
visit about:config and set privacy.trackingprotection.enabled to true.
You'll then see a shield icon to the left of the addressbar.
You can use this as an ON|OFF toggle control for the feature.
The hack has been tested and is shown to reduce page bandwidth and decrease page load times usually without breaking site functionalities. It has some advantages over Ad Block Plus and yet doesn't interfere with it either. The two can coexist.
Here's the brief PDF file from the researchers that explains the costs and benefits thoroughly: http://ieee-security.org/TC/SPW2015/...mission_32.pdf