Maintaining a LTS release while focusing on an entirely new technology with a limited number of devs available is an engineering nightmare. They have to choose their battles. The performance of the old Firefox was really bad compared to competitors and this technical debt wasn't sustainable on the long term. I'm happy that they've made this bold move (on the big risk of frustrating some users (rightfully so)), and happy that they rebuild a better browser for all of us (and I'm sure they will come back in the future with the same flexibility for addons).