As for memory usage...
For some people, it uses less memory, than FF2, for some, more. Why? Go figure.
Personally, with 2GB of RAM, I really don't pat attention to it.
Why should it matter?
You don't run out of memory, do you?
Running all bunch of programs, like right now, Firefox, Thunderbird, Winamp, Windows Explorer, and Wordpad, I have CPU usage at 10% (which is the most important), and RAM usage at 50%, so what do I care how much memory Firefox uses.