Firefox - Optimize the use of memory (RAM) when minimized
You can configure the Firefox browser to make use of the minimum memory resources (RAM) when minimized.
NB: This procedure won't reduce performance of Firefox (the default settings will be restored once you maximize Firefox).
In the address bar of your browser type: about:config
Right click on the interface and select New /Boolean.
Name the config.trim_on_minimize and set the value to TRUE.