At least to me, your question is unclear as to exactly what you are attempting to do, or get.
The most direct equivalent to ::before and ::after:
If you are wanting to actually insert content, which is what the ::before and ::after CSS selectors do, then the most direct equivalent is Element.insertAdjacentHTML(position, text). In that case:
The equivalent of ::before would be:
Element.insertAdjacentHTML("beforebegin", "<p>Additional HTML content before element.</p>");
The equivalent of ::after would be:
Element.insertAdjacentHTML("afterend", "<p>Additional HTML content after element.</p>");
Element.insertAdjacentHTML() also has options of afterbegin and beforeend which insert the HTML text just after the beginning, or just before the end, of the referenced Element.
Alternately:
You could insert nodes using Node.insertBefore(newNode, referenceNode).
For ::before it would be (insert newNode before myNode):
myNode.parentNode.insertBefore(newNode, myNode);
For ::after it would be (insert newNode after myNode):
myNode.parentNode.insertBefore(newNode, myNode.nextSibling);
Obtaining references:
If you are attempting to get a reference to the element that is earlier in the DOM, then it sounds like you are looking for Node.previousSibling. If you are looking for a reference to the element that is later in the DOM, then you are looking for Node.nextSibling.
In DOM walk order:
It is also possible that you are looking for the elements that are just before and just after the reference Node in DOM walk order. However, that is not really what the CSS selectors ::before and ::after do. However, from your mention of Page Inspector, it kind of sounds like this is what you want. If so, then you will can use a TreeWalker to walk the DOM tree.
The following should do what you want (Note: Currently untested, so might be missing something.):
//referenceNode is the node for which we want to find the elements
// before and after in DOM walk order.
//Create the TreeWalker
let treeWalker = document.createTreeWalker(document.body, NodeFilter.SHOW_ELEMENT,
{acceptNode: function(node) {
return NodeFilter.FILTER_ACCEPT;
}
},
false );
//Point the TreeWalker at the referenceNode.
treeWalker.currentNode = referenceNode;
//Get the node immediately prior to the referenceNode in DOM walk order
let thePreviousNode = treeWalker.previousNode();
//Point the TreeWalker back at the referenceNode.
treeWalker.currentNode = referenceNode;
//Get the node immediately after to the referenceNode in DOM walk order
let theNextNode = treeWalker.nextNode();
As mentioned by Nickolay, if you want the full detail that Page Inspector, or the DOM Inspector (documentation), provides then you will need to use an inIDeepTreeWalker. However, it is unlikely that you want, or need, the detail which using that Firefox specific non-standard interface provides. You only need it if you want to walk through how something like how an XUL <toolbarbutton> is constructed (not the attributes/properties, but the XBL which makes up a XUL elements like a <toolbarbutton>). For the vast majority of what you are potentially thinking about, a standard TreeWalker should be just fine.
With the exception of inIDeepTreeWalker, all of the above are standard parts of JavaScript and do not require elevated privileges (i.e do not require it to be in an add-on).
