I'm not very keen on using add-ons and any other kind extensions for their notorious ability to cause every type of corruption of browsers' functionality but this is a much needed add-on for a person that watches English-speaking YouTube videos and doesn't want any unknown word to slip through his attention for later scrutiny in order to extend his/her learning ability. Sadly more and more YT videos adopt auto-generation of subtitles which this add-on can do nothing about. Auto-generation as it comes from the notion's definition occurs at run-time only, so there's no technical option to extract subtitles stream. I was able to download a SRT file only for one video so far.