After years of using ColorfulTabs now by Mozilla's decisions with blocking of meaningful custom styling the tabs, and the hacked together Quantum compatible version of ColorfulTabs which just uses a popup and hacked Persona theme, I'm disabling it for now... In this state its completely unusable and even worse than without this addon... I'm not giving up, will check the progress, if there's any... Good luck with the development and pushing forward the needed API changes.