There are still tons of APIs missing from WebExtensions that are required for many features of Tab Mix Plus. For instance, there is no way to get click events of any kind on tabs, so things like "reload tab on double-click" are not possible. You also can't control things like whether a new "window" opens in a tab or an actual window (to implement TMP's "open popups in tabs" feature), where tabs open (to have new tabs open next to the active one), what shows in their right-click context menus, etc.
So no, we're quite a long ways away from having a real Tab Mix Plus replacement. Having said that, I have faith that we will eventually get there, and hopefully be able to implement at least half of the functionality of TMP using WebExtensions within the next two years.
You probably don't want to keep using a version of Firefox that isn't going to get security updates... If keeping Tab Mix Plus is that important to you, try Waterfox[0] instead. It's a fork of Firefox 56 with security patches backported and a few small changes.
Personally, I've installed add-ons for the bits of functionality that are possible with WebExtensions, and otherwise learned to deal with it. The performance and stability improvements are worth it.
Tab Mix Plus is pretty much essential for me. I have over 900 tabs at the moment, and all other tab extensions I've tried have been very ordinary in comparison.
So no, we're quite a long ways away from having a real Tab Mix Plus replacement. Having said that, I have faith that we will eventually get there, and hopefully be able to implement at least half of the functionality of TMP using WebExtensions within the next two years.