repo: https://github.com/KraXen72/bismuth
firefox store: https://addons.mozilla.org/en-US/firefox/addon/bismuth-color-picker/
just clone this repo and load unpacked extension. (you have to enable developer mode)
Get it on Mozilla Addons Store
Firefox version does not have a working EyeDropper API implementation, so the EyeDropper button does not work.
For now you can install another extension like ColorZilla to pick the color, and use the QuickPaste button to get it in bismuth.
Implementing a custom EyeDropper is currently out of scope, either wait for firefox to add the EyeDropper API or submit a pull request implementing an eye-dropper similar to the EyeDropper API.