I still miss Tab Mix Plus.
Having no way to properly save/manage sessions feels like I'm one crash away from losing my (way too many) tabs, although luckily that hasn't happened to me for a long time.
And no, I don't want to be told that I should be closing tabs. I have 64Gb RAM and I can search tabs by typing "%" into the url bar. Why should I ever have to close a tab if I don't want to?
It is still developed but you have to enable userchrome.js scripting support to install it.