It worked fine for me in the last months that I had it, but recently it stopped working. It started to only change the titles, but most original thumbnails were still there. I reinstalled it, and still the same. I was wondering if they were crowd sourced thumbnails, but they always just seem to be the same thumbnail as the original. Didnt change anything either in my firefox and addon settings, so don't know why it stopped working. Maybe youtube updated something recently... :(