Is this the beginning of the end of userChrome.css and userContent.css? Perhaps. To avoid unnecessary checking of userContent/userChrome CSS files at startup, Mozilla Firefox stops applying rules in these stylesheet files by default starting Firefox 69, but allows the browser to check them by enabling a pref on about:config .
When Mozilla switched to the Web Extensions API and released Photon UI with Firefox 57, Aris, developer of Classic Theme Restorer, created a userchrome.css file to customize the new user interface. When Mozilla made it difficult to customize Firefox’s UI, issues raised that they will also stop supporting userChrome.css in the future, it looks like this is actually going to happen.
If you don’t know, you can customize Firefox UI by putting userChrome.css in Firefox Profile folder, this file does not exist by default, advanced users create it to adjust Firefox UI to your liking.
Likewise, the userContent.css file is used to customize the appearance of the site in Firefox. A few days ago, Mozilla identified The userContent.css file is being loaded in the content process and as Firefox also checks the initialization of these CSS files in the user profile.
To avoid this, the company made the decision to stop loading “Legacy User Profile Personalization Style Sheets” from version 69, but a pref is landed and will be set to true in Firefox 68, users need to manually enable the pref in Firefox 69.
The summary is to enable userChrome.css and userContent.css in Firefox 69 or later, the user needs to visit about:config and change the preference value below to “true” manually. As stated above, Pref will be enabled by default in Firefox 68, so those who update these CSS files to version 69 will not be changed.