This feature isn't yet supported in the new debugger. If you need to use it, you can, for now, switch back to the old debugger by visiting about:config and setting the "devtools.debugger.new-debugger-frontend" preference to false.
To see the docs for the old debugger, please visit Debugger (before Firefox 52).
If you hover over a DOM node in the Variables pane, it will be highlighted in the page.
Also, a target icon will appear next to the variable. If you click on this target, the Inspector will open with this DOM element selected.