When it works it's awesome, one of the most useful extensions. But when it doesn't work, it just destroys everything. I had over 200 tabs across 15 or so groups. I opened firefox today and all the tabs are gone. The groups are there but they are empty... and this isn't the first time this has happened. So if you want to organize your tabs you can use it, but at some point it's just going to break and you will waste a lot of time opening tabs again.