There is this awesome website called FirefoxCSS Store, where you can find many custom CSS scripts for your Mozilla Firefox browser.
With custom CSS, you can customize the appearance of your Firefox Browser. And this website at firefoxcss-store.github.io gives you a nice collection of different and popular Firefox Themes.
If you like a CSS, you can just click on it, where you get redirected to the specific download site to get the CSS code. After that, just put it in your userChrome.css in your Firefox profile and restart your browser if needed.
With MaterialFox, you can make your Firefox browser look like Google Chrome, or make it look like Microsoft Edge with EdgeFox. Or use something completely unique.