On Nightly it's even better, it's aggressively compacting memory, it was running around 30% (2GB total) instead of 40+ a few weeks ago. Kudos for their effort. I wish they could replicate a tiny bit of it toward the GUI.
If you don't know already, a huge UI overhaul is planned to land sometimes between Firefox 25 or 26 [0]. As far as I know, better performance is a big reason for the change as well.
I didn't know they were trying to fix performance too. To me this was only a UX redesign. Which I tried not long ago, and I was disappointed, it doesn't add much actually. A few theme changes here and there, a nice configurable menu that personally I'll never use. I'm not sure what to think about it ... we'll see.
[0] https://people.mozilla.com/~shorlander/ux-presentation/ux-pr...