I have to say that I too have started to notice that Firefox isn't the svelte, lightweight IE-fighter it once was, it seems closer to being a overweight Rocky looking to for another round in the ring. I love Firefox, it's great, just look how it forced MS into releasing IE7, introduced tabbed browsing, integrated and plugins to the masses, but when it's eating up 150Mb RAM (which I've seen on my own system), things need to change.
Yes I like many of the little extras, the ability to add search engines, and use plugins to increase my productivity and have inline spell checking. I just wish it could do all that without eating up resources and then hanging onto them. It should be more than capable of doing that without pointing the finger at extension builders. It should also allow me to remove functionality I don't want or need, like RSS feed support. Maybe they should release a Firefox and a Firefox Lite, one to act as your online OS (which is what it seems to be trying to become) and one for speed browsing.