More: Firefox’s tracking cookie blacklist reduces website load time by 44% ( May 25, 2015)Firefox's Tracking Protection feature, which blacklists the downloading of tracking cookies, reduces the loading time of the world's 200 largest websites by an average of 44 percent, according to a study carried out by a former Mozilla engineer.
Tracking Protection, which has been available since the release of Firefox 35 in January, is turned off by default.
... and here is how to turn on tracking protection:
Tracking Protection in Firefox | Firefox Help