firefox-userchrome.css

- paste this in your '.mozilla/firefox/xxx.profile/chrome/userChrome.css':

@-moz-document url(chrome://browser/content/browser.xul) {
    /* Hide Firefox top tab bar */
    #TabsToolbar {
        visibility: hidden !important;
    }

    /*  Move navigation bar up to fill tab bar space */
    #nav-bar {
        border-width: 1px !important;
        border-style: solid !important;
        border-color: transparent !important;

        margin-bottom: -1px !important;
        margin-top: -29px !important;
        margin-right: 0px !important;
        margin-left: 0px !important;
    }

    /* Navigation bar color */
    #nav-bar {
        background: #2b2e39 !important;
    }

    /* Url bar color */
    #urlbar {
        background: #404552 !important;
    }

    /* Url bar autocomplete menu color*/
    #PopupAutoCompleteRichResult {
        background: #2b2e39 !important;
    }

    /* Hide TreeStyleTab header */
    #sidebar-header {
        display: none;
    }

    /* Hide TreeStyleTab splitter */
    #sidebar-splitter {
        display: none;
    }

    /* Https lock icon */
    #connection-icon {
        fill: #bac1c9 !important;
    }
}

- paste this in your treestyletab -> advanced -> extra style rules (also, enable the 'no decorations' theme) :

#tabbar {
    background: #2b2e39;
}

.tab {
    height: 23px;
    margin-bottom: 1px;
    background: #404552;
    border: 0px;
    color: #c5c8c6;
}
.tab .label {
    line-height: 1.15;
    margin-bottom: 1px;
    margin-left: 0.25em;
}

.tab.active {
    background: #2b2e39 !important;
}

.tab:hover {
    background: #2b2e39;
}

.tab .closebox {
    display: none;
}

.newtab-button-box {
    display: none;
}