Type about:config in the address bar do the things below:
1. Fetch only pages that you click
In the about:config list, find the key that says network.prefetch-next. It should be set to TRUE. Double click it, and it will turn to false.
2. Limit the RAM usage
Select browser.cache.memory.capacity and adjust the value depending on the amount of RAM memory you have installed. For RAM sizes between 512MB and 1GB, start with 15000. For RAM sizes between 128MB and 512M, try 5000, and you will be happy of the result.
3. Reducing the RAM usage even more when Firefox gets minimized
Right click your about:config page, select New and click Boolean. A box will appear and you will have to enter config.trim_on_minimize as value. The boolean value should be set to TRUE.
4. Make pages load faster
Search for the key that says network.http.pipelining and set it to TRUE. You may alter the key below (network.http.pipelining.maxrequests) and change it into a higher value - 10 for example.
To read the whole article, click here.