Google added site isolation to Chrome in mid-2018 and since then we have been waiting for Firefox to catch up. Firefox declared their intention almost 2 years ago, but it needed a major rewrite as it splits Firefox into separate processes. Firefox are almost done, and the facility will soon be moved to the stable release channel.
By enforcing the same-origin policy, browsers can already prohibit websites from accessing each other’s data. However, mistakes happen, security bugs occasionally arise, or new and creative ways are found to bypass these permissions.
Site isolation adds another level of security and the more the better. Hopefully, if you use Firefox it is set to update automatically.