Access the configuration of Firefox by typing about:config on the address bar and then press [Enter]

add this (New .. boolean)

config.trim_on_minimize
Set it's value as true to have Firefox release as much memory as posible back to the system whenever you click to minimize. This doesn't work for "Show Desktop" (Window+D) or "Minimize all" (Window+M). Every program should have this feature by default. When I minimize a program, that generally means I'm on my way to another program. Releasing used memory makes sense since it becomes available for other programs to use.

I like to manually set this item since I almost never go back more than one page. Reloading doesn't take that long if clicking "back" is required. (You can also do some fun things with "History" in Firefox).

browser.sessionhistory.max_total_viewers 1 instead of the default -1

 

 

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