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.