$.post("/Foo", { id: "2" });

This will make Firefox state "no element found".

If on the server side there is:

[HttpPost]
public void Foo(int id)
{
    // Nothing
}

However, if I switch the 'void' to 'bool' and return true, then it works:

[HttpPost]
public bool Foo(int id)
{
    return true;
}

Now Firefox does not say "no element found" anymore.

It seems that jQuery requires/expects a value to be returned by the server for the POST request.

However, the server already returns HTTP 200 OK which should be enough.