should not here be

if (!proto.isObjectOrNull())

Then that check would always be true or always be false, so useless, right?

https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/JSAPI_reference/JSObject
https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/JSAPI_Reference/JS::Value

and no. proto can be declared after the return.

In that case proto variable is useless and can be removed :)

Note to self: proto != protoVal. The prototype property is still one that should always exist, but it seems ok to keep the safeguard as you did in D1817, especially since Script_RegisterComponentType is called only once per script component type.

JavaScript only has one construct: objects. Each object has a private property which holds a link to another object called its prototype

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Inheritance_and_the_prototype_chain

until an object is reached with null as its prototype. By definition, null has no prototype, and acts as the final link in this prototype chain.

(so the safeguard can be triggered by passing the empty object as the prototype constructor in JS, which can be considered an error by design rather than something to support)

As mentioned in D1817 the GetProperty(ctor.get(), "prototype call is redundant to the introduced one below, the isObject check is assumed true in this place, and Null also shouldn't pass.

I don't see when this may ever be Null:
https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/JSAPI_Reference/JS::Value
also read a bit of the SM code but not everything involved.

.setcan only yield in MOZ_REALLY_CRASH afaics

This seems to have been unused from the start.