Rated 5 out of 5 stars
I am upset! I love this extension, and now with Firefox Quantum, I can't use it anymore!!! :(This review is for a previous version of the add-on (1.5.9).
Use custom CSS code for Firefox 57+ ( https://github.com/Aris-t2/CustomCSSforFx/issues/2 ) to keep classic ui features or switch to Waterfox browser ( https://www.waterfoxproject.org/ - a Firefox fork with support for legacy add-ons) to keep classic add-ons.