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userChrome.css customization only gets applied on Xorg or Xwayland (adds a border and not functional fullscreen/minimize/close buttons)
tested in firefox, firefox dev edition and librewolf
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- What exactly is your css trying to customize? Might help if you post it.
- What desktop environment / compositor are you using?
If you're trying to customize the title bar, that likely only will work with client side decorations, so if you use a compositor that supports server side decorations (e.g. mostly anything besides mutter and weston), and have the "Title bar" checkbox in the "Customize Toolbar..." settings page enabled, that might be the issue.
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