Provide a supported, documented Firefox UI customization API

[phyck]

Firefox has historically allowed extensive UI customization through userChrome.css, but this mechanism is neither a supported API nor a reliably documented one. It is powerful enough that many users depend on it, yet unstable enough that UI redesigns can silently break customizations or force users to reverse-engineer Firefox’s internal CSS.

The current situation creates the worst of both worlds:

• Firefox permits UI modification.
• Mozilla provides no stable contract for it.
• Internal selectors, variables, structure, and preferences can change without a documented migration path.
• Community documentation becomes outdated after redesigns such as Nova.
• Users must inspect source code or rely on scattered, sometimes contradictory community advice.
• Accessibility and power-user workflows can break after routine updates.

Mozilla should choose a clear and user-respectful path by introducing a supported Firefox UI customization API.

At minimum, this should provide:

1. Stable customization targets for major UI components, including tabs, tab bars, toolbars, sidebars, menus, and browser controls.
2. Documented appearance properties, such as tab shape, corner radius, spacing, height, density, colors, and visibility.
3. Explicit support boundaries, clearly distinguishing supported customization from unsupported internal CSS.
4. Versioned documentation tied to Firefox releases.
5. Deprecation notices and migration guidance before changing or removing customization targets.
6. Built-in settings where appropriate, rather than requiring users to edit internal CSS for common preferences.
7. A supported extension or theme mechanism for more advanced customization.
8. Automated compatibility testing, so Firefox changes can identify which supported customizations would be affected.

This does not require Mozilla to freeze the entire Firefox UI or preserve every internal CSS selector forever. It simply requires Mozilla to define and maintain a stable layer for the parts of the interface users are intentionally allowed to customize.

If Mozilla does not want to support arbitrary userChrome.css, it should either replace it with a documented customization system or clearly retire it. Leaving users with a functioning but officially unsupported modding layer makes Firefox appear customizable while shifting all maintenance costs onto users and volunteers.

Firefox’s openness is a major advantage. A documented UI customization API would make that advantage practical rather than forcing users to reverse-engineer the browser after every redesign.

Please consider this for Firefox desktop, especially in light of the Nova redesign.

[kelimuttu]

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[kelimuttu]

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