You can't really use the highlighter from the Debugger directly yet. We have a bug open (https://bugzilla.mozilla.org/show_bug.cgi?id=653545) to make the highlighter more generally-available to our other tools.
If you have a unique selector, you can use the command line (Shift-F2 to open the Developer Toolbar) to inspect an element via:
inspect unique-selector
We intend to make DOM objects highlightable everywhere in upcoming versions of the Firefox Developer Tools.
edit - This feature has been landed and now works from the Variables View and the Console. Landed in March of 2014 in Firefox 30.