It's a good addon. I can't use firefox without it. However the recent update seems to be greying out some (but not all?) discarded tab favicons when I already have something to display that they've been discarded.
I tend to recognise tabs in part by favicon colour; is there a way to disable this behaviour? Nothing in the options for Tree Style Tab. Recent updates mentions some 'secret options' and I've no idea how to even access those.

Besides that, Tree Style Tabs sometimes loses the highlight of the active tab, and the only way you can identify it is mousing over all tabs and checking which one doesn't highlight.