bindings.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE bindings>
<bindings xmlns="http://www.mozilla.org/xbl">
    <binding id="tabs-scroll" extends="chrome://global/content/bindings/scrollbox.xml#scrollbox">
        <handlers>
            <handler event="wheel"><![CDATA[
                // Preserve original behaviour if Ctrl is held
                if (event.ctrlKey) return;

                gBrowser.tabContainer.advanceSelectedTab(
                        event.deltaY > 0 ? 1 : -1, true);

                event.stopPropagation();
            ]]></handler>
        </handlers>
    </binding>
</bindings>

userChrome.css

.tabbrowser-arrowscrollbox > .arrowscrollbox-scrollbox {
    /* Place bindings.xml in the same folder as userChrome.css */
    -moz-binding: url("bindings.xml#tabs-scroll") !important;
}