- Make sure you have the latest version (currently 18.104.22.168)
- Enter about:config in the address bar.
- Right click, add Boolean: config.trim_on_minimize (true)
- Right click, add Integer: browser.cache.memory.capacity (32768)
- Restart Firefox.
- If things get sluggish, minimize the window and restore it.
Using a tool like CachemanXP can quickly show how well the "trim_on_minimize" setting is working.