PSA: Firefox v69 users will have to set a pref to enable userChrome.css and userContent.css - FirefoxCSS 　
https://www.reddit.com/r/FirefoxCSS/comments/brmi8v/psa_firefox_v69_users_will_have_to_set_a_pref_to/

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 truein about:config.

Check out the Bugzilla comments on what's happening and enabling customizations.

>toolkit.legacyUserProfileCustomizations.stylesheets
mozillaのprefの扱いが『legacy』…？『legacy』…あっ（察し）