Hide firefox top toolbar

userChrome.css

/** 
 * Put in your profile directory
 * Which you can find through `help > more troubleshooting information` then clicking "Open profile"
 * create a directory "chrome", then a file "userChrome.css", and paste the below.
 * You'll also need to go to `about:config` and set `toolkit.legacyUserProfileCustomizations.stylesheets` to `true` 
**/

/** hide actual tabs **/
#TabsToolbar > .toolbar-items {
    display: none;
}

/** hide spacer **/
#TabsToolbar > .titlebar-spacer {
    display: none
}

/** The minimize, maximize, close buttons **/
#TabsToolbar > .titlebar-buttonbox-container {
    position: absolute;
    right: 0;
    top: 0;
    min-height: var(--tab-min-height);
}


/* leaves space for the window buttons, we're assuming they're 22px in width */
#nav-bar {
    margin-right: calc(22px * 4.5);
}