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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);
}