S i X h E a D { ? }

Type the following into your address bar in Firefox:

about:config

You will want to right click anywhere in the window, choose New, Boolean, and enter in the following text:

config.trim_on_minimize

You will want the value to be set to True in order for this to work.

credit:
http://www.howtogeek.com/howto/windows/fix-for-firefox-memory-leak-on-windows/