Thank you, this can replace my own plugin I've hacked together some years ago. (Not published, was mainly a learning exercise.)
The most frustrating thing is: my plugin still works fine. But I cannot easily enable it permanently any more. I can temporary load it. But my own Firefox doesn't allow me to trust myself permanently. Maybe I'd have to submit it for signing or use some special version of Firefox... can't be bothered. It feels so disempowering.
Ha, just a couple weeks ago I wrote a small GUI frontend for youtube-dl, to download a video with chapters and create a different .mp3 or .mp4 file per chapter (useful for music compilations). I created it with the express goal of using it with Open With. It's a very useful extension, and I've used it with youtube-dl for ages.
Pretty sure like 1% of add-ons are actively monitored by Mozilla, and using the username chosen to criticize free software is ridiculous. For example, darktrojan is a member of the Thunderbird core team, see https://wiki.mozilla.org/Modules/Thunderbird
Can't get it to work on macOS, having tried all possible commands. There seem to be open issues about it in openwith repo. But definitely seems like a good option. Thanks!