While upgrading to firefox 69, the css syntax to configure firefox
seems to have been changed (once more..... programmers think that
people have time to waste!) .
(Users think programmers change things just to aggravate them!)
From the version 69 release notes:
"Firefox no longer loads userChrome.css or userContent.css by default improving start-up performance. Users who wish to customize Firefox by using these files can set the toolkit.legacyUserProfileCustomizations.stylesheets preference to true to restore this ability."