Еще по Firefox со Stylish

#TabsToolbar tab:not([selected="true"] ) {
      color: rgba(0,0,0,.3) !important;}

#TabsToolbar .tabbrowser-tab:hover:not([selected]) {
    border-color: rgba(224,224,224,100) !important;
    border-top-color: rgba(236,236,236,100) !important;
    background-color: rgba(236,236,236,100) !important;
    color: rgba(0,0,0,.55) !important;
}

#TabsToolbar .tabbrowser-tab:hover[selected] {
    background: #0000ff !important;
}

#TabsToolbar .tabbrowser-tab:hover:not([selected]) {
    background: #0000ff !important;
}

.chromeclass-toolbar-additional{border:0 !important;}