I use this as a replacement for the discarded panorama feature. Tree structure proves to be handy or even extremely useful. A spot-on use of the lateral panel, making for a perfect integration.

Still sketchy in nightly (currently 61) with new windows popping up when dragging tabs. Otherwise, quite stable and reliable. [Edit] As of Nightly 61, no tab dragging issues after several hours testing in 61. So I was wrong, nothing sketdchy about it :). Awesome!

I would still browse pre-quantum if it wasn't for this. THanks indeed.

Seems to break the lateral panel or rather lock it down. I installed an experimental feature (notes) which uses the side panel as well. NO dice. I suppose it's worth looking into it here :
https://testpilot.firefox.com/experiments