The Mozilla Foundation has announced the deployment of Total Cookies Protection by default.

The Total Cookie Protection (TCP) feature is not new: it is already active when you use the ‘strict’ mode of the enhanced Tracking Protection. The option can be activated by going to Settings > Privacy & Security. With this announcement Firefox becomes the first browser to offer full protection against cookies tracking by isolating sites… which is another step forward in privacy and security for users.
Your data matters. Don’t let just anyone track you

Mozilla explains how TCP works as such:
Total Cookie Protection works by creating a separate “cookie jar” for each website you visit. Instead of allowing trackers to link up your behavior on multiple sites, they just get to see behavior on individual sites. Any time a website, or third-party content embedded in a website, deposits a cookie in your browser, that cookie is confined to the cookie jar assigned to only that website. No other websites can reach into the cookie jars that don’t belong to them and find out what the other websites’ cookies know about you — giving you freedom from invasive ads and reducing the amount of information companies gather about you. […]

Today’s release of Total Cookie Protection is the result of experimentation and feature testing, first in ETP Strict Mode and Private Browsing windows, then in Firefox Focus earlier this year. We’re now making it a default feature for all Firefox desktop users worldwide.