Like many other commenters, the possibility of having tabs
1) on the side
2) ordered as a tree
is one of the main draws of Firefox (privacy enhancements and extensions being another).

However, in Firefox 57 on MacOS, the performance with the add-on seems worse than with the native tabs UI, both at startup (when displaying my 100+ tabs) or when I opend many tabs in a short span of time.

The problem could well be due to the WebExtensions API that it uses and not the add-on itself, but it'd be great if you could confirm that you have thought about optimizations, piroor.

But most importantly: thanks for your work! :)

PS: I also had an issue a month ago, not long after upgrading to v57, where the extension would permanently use up a lot of CPU (the load went down as soon as I disabled it, so the causation link was pretty clear). I ended up solving it by saving all my tabs as bookmarks, clearing the tabs, then restoring them: the previous tree must have been "corrupted" in some way.

Just upgraded to ff 57, every thing is working great except:
1) ff tab bar not hidden
2) when collapsing a tree it overlaps with the tabs and trees above it. I have to keep all the trees expanded now when using the Metal theme!
---
So for those having the same problem with the metal theme, adding this fixes it:
.tab.collapsed {
margin-top: -24px;
}
.tab {
height: 24px
}

great but now i can't use it on newer firefoxes like 52+

It makes FF57 usable at least(but still missing much functionality). Is there any chance we'll be able to remove default tab bar after installing this addon? Is there webexternsion api to do this? Another thing is that I cannot find a way to move tabs to right instead of left side of the window...