Firefox is a great browser, but it costs a lot in memory usage. It's not uncommon to see 50,000 KB of memory in use. While that can't be fixed completely, here's a way to cut down on memory usage when it's minimized:
1. Open a new tab.
2. Enter about:config into the Address Bar.
3. Right-Click in any empty space. Create a New Boolean.
4. Enter config.trim_on_minimize as the Boolean's name.
5. Set it to true.
Close and restart Firefox. You'll notice a sharp decrease in memory usage:
Maximized:Minimized: