Greasy Fork

Doesn't work on latest firefox browser on windows 10

Did you get an API Key and enter it into the script?
Because if you didn't it's clear why it doesn't work. The API-Key is used to ask YouTube's APIs for the exact time and whether it is a Video, Livestream or Premiere.
In the description of the script it states: "The API key must be created at https://console.developers.google.com/ and entered into the YT_API_KEY constant in the script (YouTube Data API v3)."