A tip I would add to this article is that Firefox natively supports sidebar tabs now without needing hacky extensions. Go to about:preferences under the Browser Layout section of the General tab, and select Vertical Tabs. The tab group functionality along with Multi-Account Containers are a lot more useful under this layout IMO.
A vertical tab list with tab groups is infinitely better than anything Chrome offers (and is indeed how I have Edge configured on my work PC), but Tree Style Tab remains Firefox's killer feature for me.
Thankfully, Firefox's decision to integrate its vertical tabs into its existing "sidebar" feature, combined with the ability to shrink the sidebar to nothingness, means that I'm now able to completely "remove" Firefox's native tab list, leaving only Tree Style Tab.
I think with UserChromeJS you can run TabMixPlus too. I don't remember if it needs anything else. (Or a fork like Waterfox, of course.)
But like, if we're turning signature checking and sandboxing off, we're getting pretty far from stock Firefox. And of course, they can (and often will) break it on every update.