Mozilla developers disable file processing by default and , allowing users to override the design of websites or the Firefox interface. The reason for the default disabling is to reduce browser startup time. Users rarely modify behavior through userContent.css and userChrome.css, and loading CSS data consumes additional resources (optimization removes unnecessary disk access).
To restore processing of userChrome.css and userContent.css, the setting "toolkit.legacyUserProfileCustomizations.stylesheets" has been added to about:config.
The change is scheduled to be implemented in the Firefox 69 release, planned for September 3. In Firefox 68, an additional check will automatically enable the option "toolkit.legacyUserProfileCustomizations.stylesheets" if one of the aforementioned files is present in the profile directory. Therefore, users who already use userChrome.css or userContent.css will not need to make any manual changes.
Source: opennet.ru
