I'm using Firefox 90 on Linux (Fedora 33) and MATE Desktop. Before I upgraded to Firefox 90, I set widget.non-native-theme.enabled to false to revert the scrollbar appearance to the system theme, as per this question: [https://support.mozilla.org/en-US/questions/1342934 Scrollbar appearance changed after Firefox upgrade]. It worked on Firefox 89, but as soon as I upgraded to Firefox 90, the scrollbars went back to the non-native appearance. After the upgrade, the widget.non-native-theme.enabled is still set to false, but apparently ignored. However, I noticed that the other widget.non-native-theme.gtk.scrollbar.* settings that allow scrollbar appearance customization inside Firefox are ignored as well. If I set widget.non-native-theme.enabled back to true, then the other scrollbar settings work as expected. This tells me that widget.non-native-theme.enabled does work, but the system scrollbar theme is not picked up as expected. This looks like a bug to me.