If you’ve ever looked at your Windows Task Manager after using Firexox for a few hours without closing the application down, you’ll see that a huge amount of memory has been utilized by Firefox. Now the reason this happens is that Firefox, by default, keep the last 50 websites you visited in memory to give you a faster access time.
This is great if you have lots of RAM and tend to go back to the same websites several times each day. But if your browsing behavior does not require this memory intensive function or if you are plain short on RAM you can change the default value.
In your address bar type this:
Right click to modify the value. Restart the browser.