Add a native toolbar auto-hide toggle to Firefox

[John2022FR]

I would like Firefox to include a native toolbar auto-hide toggle.

The idea is simple: provide a built-in way to hide the browser toolbars automatically, while keeping them easy to reveal by hovering the top of the window.

Suggested behavior:
- a toolbar button to enable or disable toolbar auto-hide;
- a Tools menu entry;
- a keyboard shortcut;
- automatic restoration of the toolbars on hover;
- native fullscreen behavior preserved;
- no need for userChrome.css or userChromeJS workarounds.

Why this would be useful:
- it gives more vertical space to web content;
- it is useful on laptops and smaller screens;
- it helps users who want a cleaner browsing workspace;
- it avoids relying on fragile internal Firefox UI selectors;
- it would make an existing power-user customization available in a clean, supported way.

I created a working proof of concept using userChromeJS/userChrome.css:

https://github.com/John2022/firefox-auto-hide-toolbars

This proof of concept adds:
- a native-looking toolbar button;
- a Tools menu entry;
- a Ctrl + F11 shortcut;
- toolbar auto-hide outside fullscreen;
- native Firefox fullscreen behavior preserved.

This is not a request to support this specific script. The script is only meant to demonstrate the workflow and the usefulness of the feature.

A native implementation would be more stable, safer, and easier for users than relying on userChrome modifications.

[Jon]

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