A good idea and clean job!
Maybe the alternative to the defunct "FirePath" for CSS.
Just some improvement need to be better:
1 - Adding "Inspect with devtools-highlighter" in the menu of a page, like we have "Inspect this element".
Same in the dev tool, when we are in the inspector panel (the HTML panel)
2 - Add a toggle view button to choose how displaying the result:
Just a "result list" or a traditional "HTML" view with matched results highlighting.

This should be embedded in the element search engine in Inpector, you would only need an additional "highlight search" button next to the search engine.
It is worth you to report it as an idea on the bugzilla, like the author.

I'm don't know why this is not the core of FF :)

Concise and effective.