I don't mind firefox using 100MB ram (as it is right now, with 14 tabs opened over two windows). and I'm temporarily on 256MB ram, do not feel like I lack ram with browsing + IM + viewing videos, stuff. (256MB sucks for games though)
the browser is a major piece of software, where I get overloaded by information, it stores the rendering of quite a number of bloated pages with lots of pictures, caching etc. I don't mind it taking 40% of my small RAM (will be 10% when I get back to a huge quantity of RAM, will have 1GB sdram when I get some mobo from ebay)
to compare things, my Windows also takes about 100MB ram on boot (with a few tools in the background)
also, I don't have any need for spyware protection on my PC since I started using FF when it was called Phoenix 0.5