https://www.bleepingcomputer.com/news/security/mozilla-starts-rolling-out-site-isolation-to-all-firefox-channels/

Mozilla has started rolling out the Site Isolation security feature to all Firefox channels, protecting users from attacks launched via malicious websites.

Until today, Site Isolation could only be enabled by users of Firefox Nightly, the release channel used by Mozilla to test new features not yet ready for a wider rollout.