Is this the beginning of end of userChrome.css and userContent.css? May be. To avoid unnecessary checking for userContent/userChrome CSS files at startup Mozilla Firefox to stop applying the rules in these stylesheet files by default starting Firefox 69 but allows browser to check for them by enabling a pref in about:config.
When Mozilla switched to Web Extensions API and launched Photon UI with Firefox 57, Aris, developer of Classic Theme Restorer created an userchrome.css file to customize new UI. When Mozilla made it hard to customize Firefox UI, questions raised they’ll stop supporting userChrome.css too in future, seems that is actually going to happen.
If you don’t know, you can customize Firefox UI by placing userChrome.css in Firefox Profile folder, this file doesn’t exist by default, advanced users create it to tweak Firefox User interface to their liking.
Similarly, userContent.css file is used to customize website looks in Firefox. Few days back, Mozilla has spotted userContent.css file is loading in content process and since Firefox also looks at startup for these CSS files n user profile.
To avoid this, company has taken a decision to stop loading “legacy user profile customization stylesheets” 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 versions, user need to visit about:config and flip the below preference value to “true” manually. As said above, the Pref will be enabled by default in Firefox 68, so those upgrade with these CSS files to version 69 will see no change.
toolkit.legacyUserProfileCustomizations.stylesheets