Firefox 69 will stop parsing userContent.css and userChrome.css by default

Mozilla Developers made a decision disable file processing by default userContent.css и userChrome.css, allowing the user to override the appearance of sites or the Firefox interface. The reason for the default shutdown is cited as work to reduce browser startup time. Changing behavior through userContent.css and userChrome.css is extremely rare for users, and loading CSS data consumes additional resources (optimization removes unnecessary disk access).

Added setting "toolkit.legacyUserProfileCustomizations.stylesheets" to about:config to return processing of userChrome.css and userContent.css in about:config.
The change is scheduled to be implemented in the Firefox 69 release scheduled for September 3rd. Firefox 68 will additionally include a check that will automatically enable the "toolkit.legacyUserProfileCustomizations.stylesheets" option if one of the above files is present in the profile directory. This way, users who are already using userChrome.css or userContent.css won't need to make any manual changes.

Source: opennet.ru