Here's a tip to reduce Firefox Memory usage when it is minimized.
1) Open the Firefox and open a new tab in Firefox, type in new tab address bar about:config and hit enter.
2) After hit, a warning message will appear. Choose on I´ll be careful, I promise !.
3) In about:config settings, right click anywhere and select New and Boolean will show to you.
4) In New boolean value, type config.trim_on_minimize and click on OK.
5) Select true and click on OK.
6) Close and restart Firefox for these settings to take effect.