Nicely takes the tabs to be stacked vertically on the left. With additional custom styles applied to the browser you can also hide the tabs in the title, so your browser has just the address bar on the top (and window buttons if on Windows), unfortunately this isn't supported in the extension itself out of the box but has to be done separately.

Amazing extension. It's so much better than any other mechanism for managing a lot of tabs. Essential if you do any sort of work that involves a lot of web searches and reading documentation, and excellent for personal use as well. I'm never going back to regular tabs. An important note is that the extension itself does not hide your regular tabs but it's easy to tweak Firefox to do so.

Probably the best productivity add on you could do for your browser!

I used to use TreeTabs by Kroppy, but support & development on this stopped abt 5yrs ago and it became very unstable (ie. I'd loose tree structures on the tab after exiting Firefox).

TST by Piro continued on and is now better / more stable than Tree Tabs ever was! (Piro- thank you for continuing the development - I've got ~1000 tabs open for various projects and this is the only way I can keep them all organized!)

Sidebery was the other option, but a weak option. This one is 10x more powerful!!
ps: let's all donate to support Piro so that we keep this Addon alive!

best possible feature for a browser. this is why i still use firefox

My favorite sidebar addon. However, the default theme is ugly and gives a bad first impression. Please make the default theme Proton or Photon because initially I uninstalled the addon before due to the ugliness and datedness of the "default" blue theme which is reminiscent of early 2000's design. It's ugly and there's no reason to make it the default. Additionally it's not clear to the user where you can choose to change the theme, one must be aware of addons -> scroll to addon and be aware there is a preferences tab.