Site Isolation has been under development since April 2018 under the Project Fission codename, with Mozilla first announcing plans to add it to the Firefox web browser in February 2019. It prevents malicious websites from accessing private data (including passwords, credit card numbers, or other sensitive info) loaded from other sites. A similar feature is also available in Google Chrome, first released as an experimental feature for Chrome 63 users in December 2017, and later made generally available in Chrome 67, released at the end of May 2018.
Source: https://www.bleepingcomputer.com/news/security/mozilla-starts-rolling-out-site-isolation-to-all-firefox-channels/