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.cssanduserContent.cssby default. Your CSS customizations are not going away; all you will need to do is set the preftoolkit.legacyUserProfileCustomizations.stylesheetstotrueinabout:config.