There weren’t any straightforward guides when I looked this up, and I even had to ask myself. But I just needed to put so and so together, get some feedback here, and voila! Hopefully this can work for you too, and could edit userchrome.css in your favorite editor, and see the changes in Firefox immediately. I tested that it works with @import url("folder/file.css");, and nested imports too (if folder/file.css contained @import url("Another folder/file.css");.