One comment about Firefox is its tendency to hog RAM. Well, sad to say, but it’s true (still not as much as the other browser :p). What’s comforting about it is that, Firefox’s customization allows you to tell Firefox not to use up too much RAM. Here’s how:

• Access the about:config page (here’s how).
• Find the browser.cache.memory.capacity key.
• Double-click it to change the value
• Adjust the value. For those with 512MB to 1GB of RAM, start with a value of 15000. For those with less (between 128MB to 512MB), try a value of 5000.
• Accept the changes and see how Firefox behaves.

Source: Our-picks