[mjg59]

> Who said anything about tabs?

If all your tabs run in the same process, any vulnerability triggered by malicious content in one tab has access to all the content in any other tab. Sandboxing the brower process makes it more difficult for that to result in taking over your entire system, but in this case merely taking over the browser is sufficient.

So no, OS-level sandboxing isn't sufficient. And if you don't understand that, you should not be making assertions about security.