Almost every Firefox thread has a comment like this. I'm no Firefox expert, but I use it a lot. The most common question asked in response is:
What extensions are you running? Can you disable some of them.
I run ABP, Ghostery, HTTPS Everywhere, Firebug. No probs wrt memory.
Wrt CPU: are you sure you didn't have flash videos playing in a few of the tabs? :)
Btw, in my limited experience the meme that Chrome uses less memory is false. It just splits it across many processes (and IIRC this was shown recently using graphs and everything) making it harder to infer in something like Windows Task Manager.
Except wrt startup time, Firefox feels as fast (or faster) than Chrome.
Firefox needs to work on two things:
1. Raw startup time
2. speed up loading of awesomebar info after startup: even a simple-minded thing like write out the entire file under a new name every few days will probably defragment the file. it seems like that file gets heavily fragmented leading to long delays to load it. I might be wrong about the root cause.
YMMV.