This will move Firefox & Thunderbird to your hard drive when you minimize it, and use less than 10MB of memory while minimized. Do the following:

  1. Open Firefox and go to the Address Bar. Type in about:config and then press Enter.
  2. Right Click in the page and select New -> Boolean.
  3. In the box that pops up enter config.trim_on_minimize. Press Enter.
  4. Now select True and then press Enter.
  5. Restart Firefox.

You can also set the browser.sessionhistory.max_total_viewers value to 0 which will prevent Firefox from caching pages for the back button.  

These steps were found on this blog post This May Help Your Firefox Memory Leak.