How to reduce memory usage in Mozilla Firefox

1. Type about:config in the Firefox address bar
2. Search for browser.sessionhistory.max_entries
3. Double click on the value 50, change it to something lower, 5 should be good.

Explanations

Averagely we surf less than 5 of the websites we previously surfed before, there is no need to usage up that much memory (which is 50 by default)

Theres More!!
http://www.adamsdvds.co.uk/tutorials/firefox/about/memory/index.php