With update 3.9.21 it just doesn't work. It doesn't show any tab in the tree when I first lunched Firefox, and left clicking the icon on the toolbar does nothing. Sometimes I managed to open again the extension by disabling and enabling it again, but tabs still won't show. I rolled back to 3.9.20 and it works well again (if memory gives me problems, I'll roll back to 3.9.19). Still, I want to give you 5 stars since I know you will fix this and I love the extension, thank you for it!

Edit: I updated to 3.9.22 and it's fixed, thanks!