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!