This addon is a good start but is not sufficient, because when the user have many tabs opened, a symbol for unread tab is not clear enough to view.
I suggest to add a colored top line (like the active one) for mark the unread tabs.
In this way, together with the code below for unload tabs, is possible have a consistence Quantum style.

/* Colored line for unload tabs */ .tabbrowser-tab[pending] .tab-content { margin-bottom: 2px !important; border-top-style: solid !important; border-width: 2px !important; border-color: red !important; }