Doesn't work. It says "There is no time yet recorded today". This is your first visit on this site today. I have at least 10 tabs on this site today. It says that a possible cause is that I didn't use my computer for enough time today, but that's not true. Whatever the duration, it shows the same thing. And on the site: "There is no time logged for this period".
This is really good addon but it slows down Firefox significantly. I had to test all my addons one by one to figure it out. Its a shame as the Chrome counterpart is fast. My usecase is heavy browsing (loads of tabs). It might be ok for light usage.