1. It not longer forces to restart my entire browser for an update, where every new tab I open says "restart before you can use your browser." I am not going to use a browser that closes my various windows full of API documentation once a week. I know I can disable auto-updates, but that is also not what I want: it should update when it is finally closed (like Chrome), but not update and then break my browser until I close it.
2. Individual threads can crash without taking the browser down. This feature, individually, is why most people switched and did not look back. As with the problem above. It has been requested since 2008 (http://forums.mozillazine.org/viewtopic.php?f=8&t=890085), and again in 2015 (https://support.mozilla.org/bm/questions/1092836).
Really, it's the same thing: closing my entire browser solution is an inhibitive inconvenience in my modern workflow, and Chrome never forces this action. Instead, it does everything it can to keep my tabs alive and usable. I will reconsider Firefox when they integrate this (IMO very obvious and critical) requirement. I don't care if it's faster, or uses less battery---if it regularly forces me to close all my tabs, it's unusable.