Follow this simple procedure to minimized memory used in firefox 3

If you keep having problems with Firefox eating up too much memory, you can add a preference that will allow Windows to swap out memory when the application is minimized.

1. Open Firefox 3.
2. In the Location Bar, input about:config.
3. Click to get past the warning message that appears.
4. Right-click anywhere on the preference list.
5. Click New and select Boolean.
6. When prompted to input a New boolean value, input the following: config.trim_on_minimize
7. Select true for the value.
8. Click OK.
9. Restart Firefox 3.


source: http://www.tech-recipes.com/rx/2921/firefox_3_windows_swap_memory_minimized/