The "sandboxing" you're referring to was very limited in scope. It essentially amounted to not giving content scripts any references to non-content objects like

The "sandboxing" you're referring to was very limited in scope. It essentially amounted to not giving content scripts any references to non-content objects like the browser chrome; poorly written extensions often broke this "sandbox", and it provided no protection whatsoever against browser exploits -- any arbitrary code execution exploit would allow an attacker to run unrestricted code on the user's system.