From the 'Memory Hog' files:

Firefox is a memory hog. There, I said it. Some of you might disagree, but the fact of the matter is that even though Firefox 4 (and soon 5) improve memory over say Firefox 3.x, it's still a real hog.

Thankfully, I'm not alone in the belief that SOMETHING must be done. Mozilla is now taking this very seriously and is starting a new effort called, 'MemShrink'. The goal of the group is to target the developer resources required to solve this nagging problem.