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!

I have been using TST for about 2 years. It is great and works great for a vertical tab setup.

This is how tabs should work, especially on widescreen monitors.

This add-on makes shopping and doing deep dives so much more convenient. Tabs that go together are automatically shown together.

And for when you don't need it, you can quickly toggle the tab bar on and off with F1

Useful, polished, heavily customizable. Overall an amazing extension.

This add-on made me change my entire browser experience, once and for all.

I was fighting with tabs for so many years. Constantly having a multitude of open tabs, fearing accidentially closing the window.

I was not very productive, having to scroll and look for tabs all the time.

This add-on was recommended to me, and while it did take some getting used to, I was finally able to conquer the vertical tabs.

Now I've move on from Tree Style Tab, to the more powerful Sidebery, but this is what made me do the jump.

Piro is a tab GENIUS

Thank you for your tireless work fighting AMO failures and mozilla defects

If I had to choose only one addon with my Firefox, it would be Tree Style Tab.