This seems to help out with the memory usage quite a bit. Generally, when you minimize a window the memory usage goes way down because that application isn’t active. Unfortunately, Firefox by default doesn’t adhere to this behavior. Here’s how to force it to.
Type the following into your address bar in Firefox:
You will see a window that looks similar to this:
You will want to right click anywhere in the window, choose New, Boolean, and enter in the following text:
To change the value you can either doubleclick, or right-click and choose Toggle. You will have to restart Firefox, but after you do, you should notice the memory usage go way down whenever you have Firefox minimized.
Update: You will want the value to be set to True in order for this to work.