Starting in Firefox 69, the browser does not load userChrome.css or userContent.css by default. Users who use these files to tweak and customize their Firefox may want to re-enable them. Thankfully, it is easily can be done. There is a special option you need to turn on the about config page.
In this video, you’ll learn the steps to Enable Loading of userChrome.css and userContent.css in Firefox. Use these steps.
01. Open a new tab in Mozilla Firefox.
02. Type about:config in the address bar. Confirm that you will be careful if a warning message.
03. Enter the following text in the search box: toolkit.legacyUserProfileCustomizations.stylesheets.
04. Set the option toolkit.legacyUserProfileCustomizations.stylesheets to True.
Now, The functionality of the external CSS files is now restored.
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