Mozilla plans to land a change in Firefox 69 that disables the loading of userChrome.css and userContent.css by default to improve performance.
The files userChrome.css and userContent.css are used to modify content of webpages or the browser itself using CSS instructions.
The option to do so is not removed but Mozilla plans to make it opt-in instead of opt-out. The organization states that not having to look for the two files on startup improves the start-up performance of the Firefox browser.
Firefox users who use the files already will have the feature enabled for them automatically to avoid disruptions to their workflows or expectations.
The preference needs to be flipped to True on new installations only starting with the release of Firefox 69.
Timeline for the change (proposed, subject to change):
The preference in question is toolkit.legacyUserProfileCustomizations.stylesheets. Here is how you change its value:
Options to load userChrome.css and userContent.css won't go away but users need to be aware that they may need to change the preference to allow the loading of these files from Firefox 69 onward.
The organization announced no plans to retire the option in the future
Mozilla landed the User Scripts WebExtensions API recently in Firefox, but it appears unrelated to the change.
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