Firefox DevTools still uses Spectrum. However, we added it probably like 6 years ago, and forked it. Meaning that we made a lot of changes to it in the DevTools source code, never keeping it in sync with the upstream Spectrum repository.
It’s, unfortunately, not available to install. You’d have to get the code, copy it, and make changes to it so it worked in your webapp.
The code for it can be seen here: https://searchfox.org/mozilla-central/source/devtools/client/shared/widgets/Spectrum.js
There’s also a CSS file here: https://searchfox.org/mozilla-central/source/devtools/client/shared/widgets/spectrum.css
I don’t know how easy extracting it and making it work standalone would be. I suspect not very. If you do attempt this, feel free to come back here and ask questions about certain parts of the code that you find confusing, maybe I (or other people here) can help.