Firefox Styles Snippets (via userChrome.css)

🚧 Work In Progress 🚧

Note: Starting with Firefox 69 you have to enable toolkit.legacyUserProfileCustomizations.stylesheets in about:config.

In 'Profile Directory' (Menu > Help > Troubleshooting Information > Profile Directory) create folder chrome with file userChrome.css.

To find and inspect browser's selectors open Browser Toolbox.

Completely hide tabs strip

#TabsToolbar {
  display: none;
}

Hide tabs strip only in fullscreen

#TabsToolbar[inFullscreen="true"] {
  display: none;
}

Hide sidebar top-menu

#sidebar-header {
  display: none;
}