Usercss

ads.user.css

/* ==UserStyle==
@name           Hide ads on specific sites
@namespace      github.com/sagebind
@version        0.1.0
==/UserStyle== */

@-moz-document domain("www.deviantart.com") {
    .da-custom-ad-box, .da-ad-high-impact {
        display: none !important;
    }
}

@-moz-document domain("www.reddit.com") {
    .promotedlink {
        display: none;
    }
}

@-moz-document domain("mint.intuit.com") {
    #TxnOfferWidgetContainer, #TopTxnOfferWidgetContainer {
        display: none !important;
    }
}

@-moz-document domain("twitter.com") {
    .promoted-tweet {
        display: none;
    }
}

firefox.user.css

/* ==UserStyle==
@name           Custom Firefox styles
@namespace      github.com/sagebind
@version        0.1.0
==/UserStyle== */

:root {
    scrollbar-color: rgba(255, 255, 255, 0.25) rgba(0, 0, 0, 0.75);
}