With Site Isolation enabled on Firefox for Desktop, Mozilla takes its security guarantees to the next level.

To enable Site Isolation on Firefox Nightly:

Navigate to about:preferences#experimental
Check the “Fission (Site Isolation)” checkbox to enable.
Restart Firefox.

To enable Site Isolation on Firefox Beta or Release:

Navigate to about:config.
Set `fission.autostart` pref to `true`.
Restart Firefox.