Firefox 15 is finally out with memory optimizations, WebGL enhancements, silent background updates, and many more.
Misbehaving add-ons will get their memory leaks plugged. This should greatly reduce memory consumption in the browser... assuming you use leaky add-ons, that is. Since I hardly use any add-ons I guess this one's not really helping me any. Read more.
On the audio codecs front, Firefox 15 officially supports Opus now. It'll be a while before we start to benefit from this, if at all.
Check out the release notes for more info.