Jesus! How many add-ons do you have? Ever think about turning some of them off? I use NoScript, AdBlockPlus, and Ghostery, and I’m running around 80 Mb (80,000 Kb). I agree that a browser should be agile, but lightweight has gone the way of the dodo!
With newer machines coming in with more than 4 GB RAM and hard drives reaching 3 TB, sure we want fast, but memory and hard disk footprints don’t much matter anymore.
Well I only have 5, most are for stopping stuff from loading like FlashBlock and Ad Blocker.
I do have a habit of never closing the program down completely, so it just keeps using more resources until it is crawling.