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

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

This fork only loads files within the directory chrome/userjs_files, which can help prevent unintentional or duplicate loading.

I thought I should bring your attention to the fix upstream. https://gist.github.com/Sporif/db6b3440fba0b1bcf5477afacf93f875
This originally broke in v62 and again in v68, should be sorted for now, but needs your modifications to work with shadowfox I think?

I took the upstream current version, added the path modification, and reencoded it, this is what I have and is currently working: (pardon the lack of a pull request, seems overkill for one line copypaste TBH ;) )

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