Credit to jmsmorris and plen for the style. Find your firefox profile directory (this is different for different operating systems, google it for yours). Find the "chrome" directory under it (create it if it doesn't exist). Get the contents of the link, and put it in a file called userContent.css in the chrome directory (create the file if it doesn't exist, append it if it's already there). Restart firefox.
For comparison, I put mine here: C:\Users\Oafijev\AppData\Roaming\Mozilla\Firefox\Profiles\vb3hgdj2.default\chrome\userContent.css .