This setting apparently only works in Windows, and it tells Firefox to reduce its memory usage when minimized to the taskbar. This can be helpful for anyone that runs multiple applications concurrently. Here’s how you do it:
1) Type about:config into the location bar and press enter
2) Right click any line to bring up a sub-menu
3) Choose “New” > “Boolean”
4) Paste this into the “New boolean value” dialog box that appears: config.trim_on_minimize
5) Click Okay
6) Select True in the “Enter boolean value” dialog box that appears.
7) Restart Firefox
Now open the Task Manager side-by-side with Firefox. Find the Firefox process and note the RAM usage. Finally, mimimize Firefox and watch the RAM consumed by Firefox drop dramatically.