First off, you can't use innerHTML as a *selector*. That is, there's no function by the name innerHTML. It's just an attribute. The same is true for textContent.
So these code fragments
innerHTML('<span class="Qu L4">firstname.lastname@example.org (default)</span>')
Secondly, the methods with an "s" after "Element" return *COLLECTIONS* of elements. Essentially arrays. For example,
will return all the elements inside of the element with the id of "loading" that have the className "msg".
Even if there is only one such element, you will still get an array with one element.
So you would need to do
to reference that one element.
Needless to say, none of that code will come close to working.
I know it's none of my business, but what is the point of this? The changes won't be visible to anyone not using GreaseMonkey. Is it only for your own edification?