Edit 2019-12-04:
The firefinder addon no longer exists, but you can use the developer console (press F12), and the $$ function to get elements matching a selector, eg. to select all divs: $$('div')
Old answer:
FireFinder does exactly what you are looking for. You can evaluate either CSS, or XPath expressions, it will list the matching elements, and also draw a red border around them.
