This is a killer feature in Brave for iOS (I don’t have an Android device but it’s probably similar) and I would love to see it in Firefox. It consists of several parts that do not necessarily have to be implemented all together:
put an option in settings to enable background audio
enable PiP to allow using other apps in the meantime while playing a video in background
introduce a basic playlist which can be used in Apple CarPlay and Android Auto
optional: add ability to save playlist items for offline use