Mozilla has introduced SmartBlock 2.0, the next version of its intelligent cross-site tracking blocking blocking tech, with the release of Firefox 90. The SmartBlock mechanism, introduced with Firefox 87 in March, works to ensure that the Tracking Protection feature and Strict Mode will not break websites when blocking tracking scripts. The same treatment is applied to all sites where you want to sign in with your Facebook account: all scripts used by Facebook for tracking are automatically blocked, but the login script will be allowed to load so that the login process still works.
Source: https://www.bleepingcomputer.com/news/security/firefox-90-adds-enhanced-tracker-blocking-to-private-browsing/