As result from the conversation on bug 1099331:

(In reply to Andreas Tolfsen ⦗:ato⦘ from bug 1099331 comment #32)

To return the HTTP status code with the navigation response?

Technically a driver provide whatever additional fields it wants,
as long as they are prefixed with "moz:". I think it would make a
lot of users happy to have access to the HTTP status code.

I even wonder if it would make sense to request it to be added to the WebDriver spec, so that every driver could benefit from.

Status codes for what? Status codes have always been out of scope for WebDriver because no one could define which status codes are useful and if they really wanted this feature they can use a proxy and generate a HAR.

Thinking more about it you are right. By loading a page a lot more resources will be loaded too, and each of those requests have a status code. So there is no way to return just a single one.