Set a folder named chrome on C:\Users\username\AppData\Roaming\Mozilla\Firefox\Profiles\xxxxxxx.default-release. Create userChrome.css and userContent.css on chrome folder.
To improve performance, Firefox 69 and later will disable loading of userChrome.css and userContent.css by default. Your CSS customizations are not going away; all you will need to do is set the pref toolkit.legacyUserProfileCustomizations.stylesheets to true in about:config.
/*remove new tab favicon*/