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.

"This add-on is not actively monitored for security by Mozilla. Make sure you trust it before installing."

Hm, well I'm sure it's by a well-known and trustworthy author.

"The source for this add-on is at https://github.com/darktrojan/openwith."

Dark Trojan. Sounds legit.

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!