On my secondary PC downstairs, the new version of Colorful Tabs got auto-installed, and it is starting to look good. The background is white, the current tab is colored, and I can read the other tabs (black text on white background).
However, on this (my primary) PC, the new version didn't install. I go to the ColorfulTabs website & request an install, I get a "Firefox prevented..." window and choose "Allow". Despite that, I get an error message stating the "Firefox prevented this site from installing an unverified add-on".
I have other reasons to dislike Quantum -- it is not backwards compatible. All my customization (e.g. the order of the buttons, putting the tabs at the bottom, etc.). So annoying.