Customize Firefox to your memory needs

Limit RAM usage:

If Firefox takes up too much memory on your computer, you can limit the amount of RAM it is allowed to us.
1. Go to about:config by typing about:config in the address bar.

2. Select "browser.cache.disk.capacity". It is set to 50000, but you can lower it, depending on how much memory you have. Try 15000 if you have between 512MB and 1GB ram.

Reduce RAM usage further for when Firefox is minimized:

This setting will move Firefox to your hard drive when you minimize it, taking up much less memory. And there is no noticeable difference in speed when you restore Firefox, so it’s definitely worth a go.
1. Go to about:config

2. Right-click anywhere and select New-> Boolean.
3. Name it "config.trim_on_minimize" and set it to TRUE. You have to restart Firefox for these settings to take effect.