Hello! Sorry for causing discomfort. Are you using Firefox Nightly? Currently, this add-on is not so compatible with it. If not, it will be hugely appreciated if you can file an issue on GitHub: https://github.com/YS-Wong/Adaptive-Tab-Bar-Color/issues, with screen record if possible. I'm still actively fixing the bugs.

Edit: there is a temporary fix for the bug. The add-on works on Nighly & DE now.