NOTE: Doesn't work with Firefox 72+. Add userChrome.js support to Firefox with just userChrome.css. A compact version of https://github.com/Sporif/firefox-quantum-userchromejs. Instead of loading userChrome.xml file it uses a data URI.

userChrome.css

toolbarbutton#alltabs-button {
    -moz-binding: url(data:text/plain;charset=utf-8;base64,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);
}

@Sporif Oh, dear, thanks for the information. Now I wonder whether there is another way to give the scrollbar button a more contrastive (darker) colour.

@Cerberus-tm There are other methods of using custom scripts: https://github.com/Aris-t2/CustomJSforFx