Following these steps you'll reduce the memory Firefox needs when it's minimze:
1. With Firefox up and running, type about:config and hit enter.
2. Right click on the new page and select New -> Boolean.
3. In the pop-up window, type in "config.trim_on_minimize" without
quotes and hit enter.
4. In the next pop-up window, select True and hit enter.
5. Restart your browser