based off of the following code https://stackoverflow.com/a/43794379/11006719 this edited version fixes the issue that would only let you use increase the volume on one video element every time the url is changed (The Firefox versions run when you click the page do to Firefox requiring the user to interact with the page before it allows you to create an AudioContext).
The versions with safety in the name (and the userscripts) will prompt you to confirm your input when you set it so high.