ff-compacted.css

:root {
    --height: 16px; --icon-size: 16px; --margin: -8px;
    --font-size: pt;
    --align: left; /* left | center | right */
}
@-moz-document url(chrome://browser/content/browser.xul) {
    #nav-bar {
        margin: -3px !important;
        margin-left: var(--margin) !important;
        height: 30px !important;
        padding: 0px !important;
        border-bottom: 0px solid !important; 
    }
    :-moz-any(#nav-bar) .toolbarbutton-1 > .toolbarbutton-menubutton-button > .toolbarbutton-icon,
    :-moz-any(#nav-bar) .toolbarbutton-1 > .toolbarbutton-menubutton-dropmarker > .dropmarker-icon,
    :-moz-any(#nav-bar) .toolbarbutton-1 > .toolbarbutton-badge-stack,
    :-moz-any(#nav-bar) .toolbarbutton-1 > .toolbarbutton-icon {
        -moz-margin-start: 0;
        padding: 0px 2px;
        border: 1px solid transparent;
        margin-top: -2px !important;
        margin-bottom: -2px !important;
        border-radius: 2px;
        transition-property: background-color, border-color;
        transition-duration: 150ms;
}
    #back-button{
        top: -10px !important; bottom: -10px !important;
        margin: -8px -1px !important;
        height: 40px !important;
    }
    #forward-button > .toolbarbutton-icon,
    #forward-button[disabled=false] > .toolbarbutton-icon {
        margin:auto !important;
        padding-top: 2px !important;
        padding-bottom: 2px !important;
        margin-right: -2px !important;
    }
    :-moz-any(#TabsToolbar) .toolbarbutton-1 > .toolbarbutton-menubutton-button > .toolbarbutton-icon, 
    :-moz-any(#TabsToolbar) .toolbarbutton-1 > .toolbarbutton-menubutton-dropmarker > .dropmarker-icon,
    :-moz-any(#TabsToolbar) .toolbarbutton-1 > .toolbarbutton-badge-stack,
    :-moz-any(#TabsToolbar) .toolbarbutton-1 > .toolbarbutton-icon {
        opacity: 0.9 !important;
        margin-right: 4px;
        border-radius: 12px;
        margin-bottom: 1px;
    }
    :-moz-any(#TabsToolbar) .toolbarbutton-1 > .toolbarbutton-badge-stack,
    :-moz-any(#TabsToolbar) .toolbarbutton-1 > .toolbarbutton-icon{
        margin-left: -2px;
    }
    #tabbrowser-tabs .tabbrowser-tab .tab-icon-image {
        width: var(--icon-size) !important;
        height: var(--icon-size) !important;
    }
    tab {
        font-size: var(--font-size);  text-align: var(--align) !important;
    }
    .tabbrowser-tab[selected="true"] .tab-stack {
    }
    .tab-stack{
        margin-block-start: -1px !important;
        margin-block-end: 4px !important;
        height: var(--height) !important;
    }
    #TabsToolbar > #tabbrowser-tabs > .tabbrowser-tab::before,
    #TabsToolbar > #tabbrowser-tabs > .tabbrowser-tab::after {
        opacity: 0.4;
    }
    #TabsToolbar{
        -moz-appearance: none !important;
        margin-left: -8px !important;
        margin-right: -8px !important;
        background: var(--bg) !important;
        height: var(--height) !important;
        margin-bottom: -1px;
    }
    #tabbrowser-tabs {
        -moz-appearance: none !important;
        margin-left: var(--margin) !important;
        margin-bottom: 1px !important;
        background: var(--bg) !important;
    }
    
    .tabs-newtab-button {
        width: calc(36px + 28px);
        list-style-image: url(chrome://browser/skin/tabbrowser/newtab-inverted.svg);
        opacity: 0.9 !important;
    }
}