According to Mozilla, Firefox is designed to use 100MB to 150MB of RAM. By design, the more RAM that is available on your system, the more Firefox will use. Here are a few things to check if you feel Firefox still takes up too much memory.
Extensions are the main culprit in using too much memory with Firefox. Check under Tools>Add-ons and uninstall any Extensions that you don’t need.
Certain themes or skins can cause problems with Firefox. Check under Tools > Add-ons and uninstall Themes except for the Firefox default.
3. Certain Web Sites
If a web site has a lot of data in it or something like a Flash movie playing constantly, it will take up a lot of memory. Close unneeded web site tabs or windows that may have this problem.