If you are worried about the memory usage of firefox, you can try typing about:config at the address bar. Right click to create a new integer and enter the preference name as browser.cache.memory.capacity and set the size you want the cache to be (in kilobytes). Then, you are done. Restart firefox for the changes to take effect. Not sure it works though but it works on me.

you can also add this trim memory when minimize..

create new boolean :

config.trim_on_minimize set the value to true

..there you go..

edit : add pic