I tried contacting the author to see if he would open source it but he's gone dark and doesn't seem to really exist on the Internet anymore. So I grabbed the source code and modernised it, added additional feature and it's what I've been using ever since. I haven't open sourced it out of respect for the original author however the software does not actually have any license that prevents it so I have been considering what the best approach might be (any ideas?).

*Edit: Whoops! I was confusing refinedHN with modernHN (https://www.modernhn.com)

Are you referring to https://github.com/plibither8/refined-hacker-news?

It's MIT-licensed open source.

I've been using a fork (also MIT): https://github.com/alexferrari88/refined-hacker-news

Just use my extension?