Hi, is not wide enough, not bright enough ?? Sorry I do not know the code for changes to that. You can make over 235 million changes to Firefox , need to learn though : USSER CHROME URL'S *https://www.howtogeek.com/334716/how-to-customize-firefoxs-user-interface-with-userchrome.css/ *http://techdows.com/2017/09/classic-theme-restorer-userchrome-css-modify-firefox-57-photon-ui.html *https://github.com/Aris-t2/CustomCSSforFx/issues/1 *https://www.accessfirefox.org/Theme-Font-Size-Changer.php *https://www.userchrome.org/ *https://www.accessfirefox.org/Firefox_Accessibility_Themes.php *https://www.userchrome.org/what-is-userchrome-css.html *http://kb.mozillazine.org/UserChrome.css *https://github.com/axydavid/FirefoxUI/blob/master/README.md *https://github.com/wilfredwee/photon-australis *https://www.reddit.com/r/FirefoxCSS/ Note not all code in one place. Please let us know if this solved your issue or if need further assistance.

Unfortunately, Firefox is a little stubborn in following your system-level scrollbar colors. You could look for a tool to change the scrollbar color throughout Windows. Be cautious with free software to make sure it doesn't have a lot of "crap" bundled with it.