From the Mozilla blog:
Tracking slows down the web. In a study by Ghostery, 55.4% of the total time required to load an average website was spent loading third party trackers. For users on slower networks the effect can be even worse.
In the physical world, users wouldn’t expect hundreds of vendors to follow them from store to store, spying on the products they look at or purchase. Users have the same expectations of privacy on the web, and yet in reality, they are tracked wherever they go. Most web browsers fail to help users get the level of privacy they expect and deserve.
I use Safari as my main browser, but I switched to Firefox as my secondary about a year ago because of their focus on performance. Ad-Tech tracking is horrible for performance, so it makes sense that Mozilla would tackle this as part of that effort. Not only will these changes increase performance, but they have the added benefit of blocking all the advertisers from tracking your every move across the web.