Get and display the total # of tabs open in a given window (firefox css)

tabcounter.css

/* get count */
#tabbrowser-tabs {
    counter-reset: tabcount;
    & .tabbrowser-tab {
        counter-increment: tabcount;
    }
}
/* VERTICAL tabs visual indicator */
#tabbrowser-arrowscrollbox[orient="vertical"] {
    #tabbrowser-arrowscrollbox-periphery::after {
        content: counter(tabcount) " Tabs Open";
        position: absolute;
        bottom: -22px;
        left: 20px;
        word-spacing: 0.2rem;
        font-style: italic;
    }
}
/* adjustment for collapsed tabs */
#sidebar-main:not([sidebar-launcher-expanded=""]) {
    #tabbrowser-arrowscrollbox[orient="vertical"] {
        #tabbrowser-arrowscrollbox-periphery::after {
            bottom: -36px;
            left: 1px;
            text-align: center;
        }             
    }
}
/* adjustment for tabs overflow */
#tabbrowser-tabs[overflow=""] {
    #tabbrowser-arrowscrollbox[orient="vertical"] {
        #tabbrowser-arrowscrollbox-periphery::after {
            position: fixed;
            z-index: 99999999999999999;
            bottom: 72px;
            left: 104px;
        }             
    } 
}
/* HORIZONTAL tabs visual indicator */
#tabbrowser-arrowscrollbox[orient="horizontal"]::after {
    content: counter(tabcount) " Tabs Open";
    margin: 12px 10px !important;
}

Preview

• Updated to include a different approach for horizontal tabs by sifferedd
• For just the number and no text, replace all instances of " Tabs Open" with ""