The video explains how to install extensions, also known as add-ons in Mozilla Firefox
Extensions are the programs that change the browser's functionality i.e. add new features or modify the existing ones positively. While most add-ons are beneficial, some are not. E.g, FF Web Surety add-on injected “Monero” miner into Firefox.
Procedure To Install:
Search for add-ons from within the Firefox page or visit the website and download the add-ons.