ClipHearth от ClipHearth

ClipHearth is a media app for your computer: it saves, converts and edits video and sound, and adds subtitles to it. This extension adds a button and a right-click menu item, "Send to ClipHearth", that hand the page you are on to the app. ClipHearth then shows what it can do with the media on that page: pick a quality or the sound only, add subtitles, keep just a part, convert it or edit it later.

• One click on the toolbar button, or right-click a page, a link or a video
• All the work happens in the app: quality, subtitles, conversion, editing and the queue
• Nothing is sent to the internet: the address of the page goes only to the ClipHearth app on your computer, and only when you click

Requires the ClipHearth app for Windows, macOS or Linux: https://cliphearth.com
The first time, Firefox asks whether to open ClipHearth. Tick "Always allow" to skip the question next time.