Using it for a long time now... I love it... Firefox should have vertical tabs by default, its way more intuitive to scroll up/down than to move the tablist left/right, and its more productive since pages are vertical usually and vertical space is more precious nowadays

The tree feature is the best grouping/tracking of set of pages

The only thing this addon lack is the coloring of the tabs, would love to see that feature in a future release

To change the size of the tabs do:
/* Change tab height */
.tab {
height: 24px;
}