Fx3 on Mac のテーマになるらしいやつをちょっとつかってみる ( https://addons.mozilla.org/en-US/firefox/addon/6050 )

けど、縦置きのタブは全く考慮されていないので (そりゃそうだけど) スタイルあてなおした

@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);


.tabbrowser-tabs * {
width: 100% !important;
}


.tabs-stack {
width: 10em !important;
padding: 5px 0 0 0 !important;
border-style: solid !important;
border-width: 1px 0 0 0 !important;
border-color: #444 !important;
background:#797979 url("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAH8AAAATCAIAAAA26UVlAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAAFlJREFUWMPt1MENwEAIA0GSrumXKjjZpIhIx2e3hLHkJzODbjUzks453V1VLyKX9WfGtqSIQH9nAPTX9G3bRn/zfNBfDn300Sf00Sf00Sf00Sf00Sf00ae/fcgdNtGP2RYEAAAAAElFTkSuQmCC") repeat-y top right !important;
}

.treestyletab-splitter {
background: #b3b2b3 !important;
border-style: solid;
border-width: 0 1px 0 0 !important;
border-color: #4e4e4e !important;
}

tab .tab-icon {
margin: 0 0 0 0 !important;
}

tab .tab-text-stack {
padding: 6px 4px 4px !important;
}

tab {
background: url("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAkAAAAaCAIAAADqseFyAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAANxJREFUKM91kk2yhCAQg3NaWeNKrzEqiPzNUV93AwpjvY8QAlT1KtCz1jOhK3PdZFDTNClak1KsgpIXxJgi78QHe7oz8otvE3LKrNx5E1KFLonuqQPfnP8bi2fIayy6Gb9jEXvScEMMsSl0mYXgA6vgB4f3wTfCbRJwuYt1XUMQpz9HekF/Du507iTE2J1rgq2cwyHAMFa2tZKtZEowh2EZU0MnHPtBokVns10eDuzb/qjQrtg+28DnCZCOcF9i7UvpDp8o9erbdlcNtZXSUj23jkrAsqxLYR1tWf8AJr2Qe2DQNzkAAAAASUVORK5CYII=") repeat-x top left !important;
border: none !important;
padding: 0 0 0 0 !important;
height: 26px !important;
}

tab .tab-image-left {
background: url("data:image/png;base64,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") no-repeat top left !important;
width: 10px !important;
}

tab .tab-image-right {
background: url("data:image/png;base64,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") no-repeat top left !important;
width: 10px !important;
}

tab[selected="true"]  {
color: #000 !important;
border: none !important;
background: url("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAkAAAAaCAIAAADqseFyAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAADhJREFUKM9jyMzKTAeCDAhGQQx+uAHD5k2bcaFRuVE5DLndu3bjQnjTWUYGKGkiJVEoBwgYKnEDAAQjv6hLimGWAAAAAElFTkSuQmCC") repeat-x top left !important;
height: 26px !important;
}

tab[selected="true"] .tab-image-left {
background: url("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAA8AAAAaCAIAAADnr5E1AAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAAPpJREFUOMuV1N1uwiAUB3CfcAXcRXspaTLpOGS6FCP1NWYfo0m/aPHlZEuNaaru8A9w9Qs5J+GwOt5iisKfWh++93sAJTLx4bOd9l+2q+M9JtcaQG44j+P47VEmXZxO+qCFEGmann/OVVU1dbNckzbGgFJZltV1Y63t2u6V1nnOOS/L0tOHbqa/drskSdqmfXbrTPsaKGWv6V1/SskY67sepUFKyqjtLUpLr2mgHuyA1YTQcRhxGoAQgtXgdUTc6JCVQBRFzjnk3dLri7tguwzTvkusVkqt1+/YusPeYNj7Dpud5Vz6jpfrpn+/h9nMM0afz3xRhP4n/+cKjYKqgQhXyaIAAAAASUVORK5CYII=") no-repeat top left !important;
width: 10px !important;
}

tab[selected="true"] .tab-image-right {
background: url("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAoAAAAaCAIAAAABhlpxAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAAEFJREFUKM9jSE/PwIMY/PAChs2bNuNBo9Kj0tST3r1rNx5EKKWmQ0EGEMCYUAwEDJWooLy8vLCwMCsrKyEhAagbALBg8EPs/C58AAAAAElFTkSuQmCC") no-repeat top left !important;
width: 10px !important;
}

色とか改訂されて使えなくなりそうだ