See video thumbnails, ratings and other details when you mouse over a Youtube link from almost any website
< Feedback on Youtube Peek Preview
You should have included how to get a key... ...
This post describes how to get your very-own "YouTube Data API v3" key: https://www.reddit.com/r/firefox/comments/cn8w11/anyalternativestoyoutubelinktitlescript/ewf8tap/
which you can then use in this script, and also the original YouTube Link Title. You can even use both together (run YLT first).
If it stops working get an API key
The script uses a YouTube API key. It seems that if enough people use it in a day it will reach the limit of its daily request quota and stop working. To get around this you will need to get your own API key from the Google developer console and replace it with the current one on line 249.