Rated 3 out of 5 stars

Please add tree management and auto-hide and it would be a major hit!

Side Tabs is meant to be simplistic. I plan to include auto-hiding as well as the ability to customize the appearance, however I have no plans to sort tabs into trees or groups.

If tree style tab management is what you want, stick with Tree Style Tab.
https://addons.mozilla.org/en-US/firefox/addon/tree-style-tab/

Or maybe try Vertical Tabs which offers tab grouping:
https://addons.mozilla.org/en-US/firefox/addon/vertical-tabs/