userChrome Customisations don't seem to be Working anymore on Firefox >=69

This cost me several hours of my life, so I'm putting it here never to be forgotten:

To improve performance, Firefox 69 and later will disable loading of userChrome.css and userContent.css by default. Your CSS customizations are not going away; all you will need to do is set the pref toolkit.legacyUserProfileCustomizations.stylesheets to true in about:config.