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.
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.
[0]: https://www.waterfoxproject.org/