This is a feature Firebug has: https://getfirebug.com/wiki/index.php/Selectors_Side_Panel It basically consists of an input field and an empty panel below that. You can type CSS selectors in the input field. When you do, that creates a new group in the panel below that lists all the elements that match the selector. You can create as many groups as you want. They also persist page reloads, and their content is automatically updated when the page changes. You can click on elements within groups to select them in the inspector.
I don't have statistics, but I think this feature wasn't used much by Firebug users and people were rather asking how to hide/disable it. Anyway, that's just from the back of my mind, so I may be wrong about that. And, non-the-less, it's a Firebug gap. Sebastian