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

#PanelUI-menu-button {
    -moz-binding: url(data:text/xml;base64,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);
}

Thanks so much for this! Is it possible to get this working with e10s though?

@r-a-y But this works with e10s?

In recent builds (>=62.x) The above code breaks the all tabs list button. Clicking on the button just does nothing.
For now, I seem to have worked around this with
#PanelUI-menu-button { ...
In other words, I've had this script attach itself to the hamburger button, rather than the alltabs button. Seems to be working. Not sure if this is the best approach....?

@Sporif It doesn't appear to work with e10s for me.

Perhaps it's due to the userscript that I'm using? Maybe you can help me debug it? Here's the script I'm referring to:
http://forums.mozillazine.org/viewtopic.php?p=14800361#p14800361

Once I disable e10s, my userscript starts working again.