Thank you! 
But I think the biggest thing is that Mozilla recognizes their performance chugs on low-spec devices. Now that they're focusing heavily on mobile, these efficiency improvements are getting into the core browser as well. 19 is already a lot faster than 17, and we're busily whacking bugs out of 20. By the time 24ESR comes out, I bet it'll be sleeker still.
But I think the biggest thing is that Mozilla recognizes their performance chugs on low-spec devices. Now that they're focusing heavily on mobile, these efficiency improvements are getting into the core browser as well. 19 is already a lot faster than 17, and we're busily whacking bugs out of 20. By the time 24ESR comes out, I bet it'll be sleeker still.