I have to disagree, Firefox is in a much better position than a few years ago. RAM usage is lower, e10s works nicely. Tab Groups was spun out into an addon; a sensible decision for a feature extremely few people used. Test Pilot is a better solution for testing features like this.

Very relevant to this thread, addons and plugins are a huge cause of poor performance - I ran into this a while back, where an addon was leaking memory via zombie tabs. Have you tried disabling addons?