> To use out-of-process tabs, you must set the layers.offmainthreadcomposition.enabled preference and restart. Opening a normal window instead.
So it does actually require a about:config tweak to enable it.
* Linux OMTC: https://bugzil.la/722012 * Windows OMTC: https://bugzil.la/913249
Now, the sandbox is also code, but to my understanding it's fewer LoC than what you are sandboxing. As bug rate per LoC is a fairly stable value, reducing LoC reduces total bug count. Ergo, by wrapping a large complex program with many LoC inside a small sandboxing function, you increase security (though it is not perfect, it still will have SOME bugs)