I too am a Tab Mix Plus fan. I was using an ESR version to try to maintain better security while still using my favourite add-on. Now seemingly not possible. I am quite prepared to put up with security vulnerabilities for my personal use of browsers. I use sandboxing/ firewalls / VM's to protect myself. Not happy that an older version of Firefox was broken. I don't expect a company to maintain older versions forever, I do expect them to not deliberately or accidentally break them.

Give me back my Tab Mix Plus!