oba pliki w user/appdata/roaming/mozilla/firefox/profiles/323jkjkkj.default/chrome/

userChrome.css
/* this changes add bookmark tree length */
#editBMPanel_folderTree{ min-height: 400px }

userContent.css
/* this changes scrollbar appearance */
:root{ scrollbar-color: rgb(100,100,100) rgb(42,42,46) }
*{ scrollbar-width: thin }