No, it's still supported.

You need to place userChrome.css and userContent.css in the chrome folder in the Firefox profile folder.

You can use the button on the "Help -> More Troubleshooting Information" (about:support) to go to the current Firefox profile folder or use the about:profiles page (Root directory).

• Help -> More Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

In Firefox 69 and later you need to set this pref to true in about:config to enable userChrome.css and userContent.css.

• about:config => toolkit.legacyUserProfileCustomizations.stylesheets => true
• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

I see nothing here about userContent.css much less userchrome.css.

What am I missing?

Possibly nothing in case you have no purpose for using userChrome.css. I'm not sure why you asked this question.

Both userChrome.css and userContent.css do not exist by default and you need to create the chrome folder in the Firefox profile folder and within this folder userChrome.css/.userContent.css if you want to customize Firefx.

• https://www.userchrome.org/what-is-userchrome-css.html

Thank you for your help. One more thing on my recreation list.

Per your link, I was able to create the “chrome’ folder then drop my userChrome file (from my backups) into it. Now it works!

Note: "Mark it as solved" does not seem to work but it is solved.

Thank you again.