They're shared among site instances, not the same domain. It has to do with web standard semantics. Google has been working on https://www.chromium.org/develope

They're shared among site instances, not the same domain. It has to do with web standard semantics. Google has been working on https://www.chromium.org/developers/design-documents/site-isolation https://www.chromium.org/developers/design-documents/site-is... for quite some time which will probably ship in 2017. That's the next step beyond the --process-per-site-instance after --isolate-extensions. Firefox is more than just a little bit behind when it comes to sandboxing though... there's a LOT more to sandboxing than splitting up processes. That doesn't result in any meaningful isolation without a lot of further work, and improving sandboxing is a major undertaking that Chrome has been working on for years.