Today, Firefox is introducing Enhanced Tracking Protection (ETP) 2.0, our next step in continuing to provide a safe and private experience for our users. ETP 2.0 protects you from an advanced tracking technique called redirect tracking, also known as bounce tracking. We will be rolling out ETP 2.0 over the next couple of weeks.
TL;DR: Mozilla were already blocking third-party Cookies, so the ad industry started making webpages redirect to their domain (so that they could set a Cookie as first party).
Now Firefox will also automatically delete first-party Cookies that are known to be tracking Cookies.
This excludes Cookies from webpages where you might legitimately be logged in, like Google or Facebook.
Thanks for the explained summary
Love TL;DR’s :)