Was great on Ubuntu, even worked on sites with tricky JavaScript-enabled videos like saddleback.com, but now (probably due to the new snap-based Firefox on Ubuntu) just says I need to install the companion app (which is already installed). Unfortunately, going to the support link here or on the plugin's home page just takes you to a Google Groups page that displays "Banned content warning". Not sure if this is Big Tech trying to squash a tool to download from YouTube or what, but it means the plug isn't useful on Ubuntu any more. :(