I like the addon and I modified the CSS Code to a more original firefox-like menu:

html, body {
margin: 0px; padding: 0px;
overflow: hidden;
}

#container {
border: 0px solid #DCDFE3;
background-color: #fff;
}

.panel {
width: fit-content;
height: fit-content;
background-color: #fff;
position:absolute;
top: 0; left: 0;
font-size: 10pt;
padding: 4px 0px;
user-select: none;
-moz-user-select: none;
-webkit-user-select: none;
overflow-x: hidden;
overflow-y: auto;
max-height: 580px;
}

table {
border-spacing: 0px;
margin: 0px; padding: 0px;
background-color: #fff;
}

td {
margin: 0px;
padding: 0px;
}

td * {
vertical-align: middle;
}

td.arrow {
padding: 0px 12px 3px 0px;
}

td.icon img {
border: none;
width: 16px;
height: 16px;
margin: 5px 5px 4px 12px;
}

td.title span {
font: message-box;
color: -moz-FieldText;
display: inline-block;
margin-right: 5px;
white-space: nowrap;
overflow: hidden;
text-overflow: ellipsis;
cursor: default;
max-width: 500px;
min-width: 125px;
margin-bottom: 2px;
}

tr:hover {
background-color: #f0f0f0;
}

div.splitter {
width: inherit;
height: 1px;
background-color: #fff;
margin: 6px 0;
border-top: 1px solid #ddd;

}
div.return_box {
position:fixed; left: 0px; width: 20px;
background-color: #fbfbfb; border: 0px solid #c0c0c0;
border-left: none;
}

div.return_box:hover {
background-color: #f0f0f0;
}

div.return_box img {
margin-left: 7px;
}

span.empty {
color: #aaa;
}