Rated 3 out of 5 stars
Been waiting for something like this. Sadly I use a dark theme and you use a white background with no options to change. And it just clashes badly.
Edit: Thanks for the link to the other version. 5 stars on it.
Site Favicon in Urlbar
You can try installing this one: https://addons.mozilla.org/en-US/firefox/addon/site-favicon-in-urlbar/
Hope this help.