Firefox_v57 - TreeTab (Top TabBar Hide) - macOS

Link: https://addons.mozilla.org/en-US/firefox/addon/tree-style-tab/reviews/ (Search: userChrome)
Link02:  https://gist.github.com/alexandru/0f8b9a845342dd919d48dccc844a7135
Link03:  https://www.itworld.co.kr/howto/249556

FireFox: 99.0.1
macOS: Monterey 12.3.1

1) about:config
- toolkit.legacyUserProfileCustomizations.stylesheets
- value true (default: false)

2) NewFile: userChrome.css
- Path: "about:support" - "Profile Directory"
- NewDiretory: chrome
- NewFile: userChrome.css

3) Write: userChrome.css
/* Hides tab bar*/
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
#TabsToolbar { visibility: collapse !important; }

4) FireFox ReStart

5) TreeTab Hide/Show - F1 key