This works for Mac or Windows.
Here is a tricks on how to reduce the memory usage when you minimize your Firefox.
1. Type about:config in Firefox address bar and press Enter.
2. Right click in the page and select New > Boolean.
3. Type config.trim_on_minimize in the box that pops up. Press Enter.
4. Select True and then press Enter.
5. Restart Firefox.
What it will do is, when each time you minimize your Firefox, the memory
usage will dramatically drops. The amount will differ per machine I'm sure.