This add-on adds tooltips to the names of CSS properties in the Inspector's Rules view.
It's strictly a proof of concept/demo, but it should work well enough for you to see what this feature could look like in the Firefox devtools.
To use it, you can just download the built add-on "mdn-tooltips.xpi".
Then open the Inspector, and move the mouse over a CSS property name in the Rules view. You should see the tooltip.