Mozilla says Firefox users will be protected against cross-site tracking automatically while browsing the Internet in Private Browsing mode. Starting with the Firefox 89 version released today, Total Cookie Protection will be enabled by default. Mozilla also rolling out Site Isolation to all Firefox channels, a security feature designed to protect users from attacks launched via malicious websites. To go into private browsing mode in Firefox, all you need to do is to open the Application Menu by clicking the button ( ) on the top right and then choosing “New Private Window”””
Source: https://www.bleepingcomputer.com/news/security/firefox-now-blocks-cross-site-tracking-by-default-in-private-browsing/