> "It's worth noting that Tree Style Tabs still exists and works really well."

YMMV. I use it but I find it buggy and slow. Once or twice a week it seems to crash completely and although it comes back up by itself, it takes a minute or so with large numbers of tabs. In the past, although not recently, I've also had trouble with it flattening my tab hierarchy when firefox is restarted, and having tabs at the wrong indentation level or no longer displayed under their parents.

Also, you can't have the bookmarks sidebar and the TST sidebar open at the same time anymore, which is a real bummer.

Excuse me, but post-e10s Firefox is not anywhere near as fast as old Firefox. It's a slow mess that freezes constantly, and the switch from XUL to WebExtensions (and the changes to the rendering engine resultant from that) have made it agonizing to even load basic HTML without CSS, ECMAScript, or XML content.

I switched to Pale Moon for performance and philosophy, and because it has worked since then exactly as I need it and want it to, and has been uniformly faster (now I can confirm on all three machines I use - the x64s at work and x86 at home), and it has given me no security concerns, I've made a 100% switch.