It's in alpha.

"Relax isn't yet ready for production, stay tuned for releases, beta version will come soon"

Hey! Relax creator here. Actually the demo version goes back quite a while. We're redesigning a refactoring pretty much of it. Should be completed soon and we'll update the demo when it's more stable :-)

Good news: it's open source! Maybe you can test it on Firefox and give you feedback in a Github issue?