We've got some new guidance from Markus on the user experience of "about:config".
The main change will be to replace text with icons, as shown in the attached mockup. The required icons are Pen, Undo, Trash, Check, Plus, and a new Toggle icon.
Given that it will reduce the surface and importance of the buttons, and the main use case of the page is to modify preferences, we should re-introduce double-click on the entire row as an alternative to toggling or starting to edit. This likely means that the triple-click interaction will be removed, while making sure that drag-to-select still works.
The "add" row that is visible when a preference name cannot be found should ideally be separated from the rest of the entries.