i started getting a HUGE load on my RAM when i'd leave firefox open...seems like it's related to the amount of tabs and/or extensions you use. anyway, i came upon this page which suggests a way to reduce this:
http://tech.cybernetnews.com/2006/03/26/this-may-help-your-firefox-memory-leak/
http://tech.cybernetnews.com/2006/03/26/this-may-help-your-firefox-memory-leak/