Firefox UI customization

userChrome.css

/* How/Where to edit userChrome.css https://i.redd.it/v7lkiqannojz.png */
@-moz-document url("chrome://browser/content/browser.xul") {
    /* hide navigation bar */
    #nav-bar {
        display: none;
    }

    /* hover the bottom of tab bar to show navigation bar */
    #navigator-toolbox:hover > #nav-bar{
        display: -moz-box;
    }

    /* disable tab flash */
    .tab-loading-burst[bursting]::before {
        animation: none !important;
    }

    /* remove 'dragging tab' margin/padding nonsense */
    #TabsToolbar[movingtab] {
        padding-bottom: unset !important;
    }

    #TabsToolbar[movingtab] > .tabbrowser-tabs {
        padding-bottom: unset !important;
        margin-bottom: unset !important;
    }

    #TabsToolbar[movingtab] + #nav-bar {
        margin-top: unset !important;
    }
    
    /* hide scroll bar */
    #content browser {
        margin-right: -12px !important;
    }
}