computer-centric curiosity

Does anyone know why bookmark icons don't show for most of my bookmarks in Firefox? It's been this way for roughly forever, and I've never understood why! (In the bookmarks sidebar, when a folder is open? Maybe five of the thirty visible bookmarks will have an icon besides them, e.g. the AO3 or LJ icon... even though fifteen of the links go to AO3 or LJ. Most of them will be little blank spaces, outlined by a dotted box.)

It's kind of frustrating, when I'm trying to find a fic that I know was on AO3, to be unable to pick out AO3 links from the heap, but mostly I'm just really curious as to why so few of them show, and why it's so random - which ones show up and which don't changes all the time!