Currently using Sidebar Tabs. I have gotten used to grouped tabs, which is something I don't want to give up. I've mapped Ctrl+Space to open and close it, which is super satisfying.
I switched from Sideberry, which was cool since it is so configurable, and that it can store and recall entire lists of links. I think this is a nice alternative to bookmarks, because it gives you a sort of link manifest, that you can even export.
Mozilla's attempt seems... lackluster in comparison, but it's still under development. So eh. Thinking of going Librefox, Waterfox or even Floorp - though that would be like going ESR.