Currently Firefox Quantum (Firefox 57 and upwards) has no provision to allow for styling of tabs. The same is being tracked in Bug 1320585
The Mozilla team has decided to not work on this feature. Meanwhile I'm trying to find a way to get it working in the latest Firefox. While this is sorted out, you may want to use the Firefox Extended Support Release (ESR) version from https://www.mozilla.org/en-US/firefox/organizations/faq/
I've just released it for Chrome though https://chrome.google.com/webstore/detail/colorfultabs-for-chrome/fageloggidhmmlofkpibpmngbimkmhdb.