This add on is great because it's the only one that does what it does on Firefox, to the extent of my knowings.
It would be perfect if the amount of time calculated could be displayed in a better place than the playlist's page description and the display should stand out more instead of hiding in the same font.
Also it would be awesome if it could count a selected number of videos into a sum of video length.
Thanks for making this!

It shows the time amidst of Playlist description, what is kinda weird. It took me 15min to find it. It could at least explain how to find it.