You could try a few things -
1. Get hold of the development release of firefox, called Deer Park Alpha 2. Its memory use is much better than the stable build. However, it is not completed yet so use it at your own risk.
2. Try getting an unofficial build of firefox which is optimised for your processor, and which disables a few features. Stipe's and Moox's builds of firefox are quite popular under windows. Check this for info.
3. Get the sources and compile it yourself, and tweak it for better performance. You could disable many inbuilt extensions, disable features like the java script console, DOM inspector etc., compile it with a different toolkit (under linux, the GTK+1 mozilla builds are way faster than GTK+2, however, on windows, the default win32 will do fine). Check the mozilla build
instructions.
However, even after this mozilla/firefox is not as great in speed as kmeleon/opera. I think Firefox 1.5 will go a long way in improving that
By the way, there was a discussion at the FreeBSD mailing list about the possiblity to disable XUL in firefox, which could increase interface speed a lot. As far as I understant, XUL provides the user interface for firefox, as well as the interface for extensions. So I am not sure what disabling this would do. If anyone knows if this is possible, and what are its effects, do let me know