https://epiverse.co/ shows HN and Reddit discussions for the page you are on.

Hey! I built https://discussions.xojoc.pw/ just for that. It also has a bookmarklet to search for Reddit threads for the page you're visiting.

Kiwi Conversations is an extension that does it, I use it all the time.

Years ago I had found an extension that did this. Can't remember the name, but I loved it until a reddit API change broke it, and the dev had seemingly disappeared.