How does it work? Easy. When you get to a maximum amount of memory used (the default is 500 megabytes), the extension automatically restarts Firefox, saving the tabs you had open at the time.
This way, you can stop worrying about having too many active tabs at once and slowing Firefox down.
Memory Restart is a super useful add-on for Mozilla Firefox. With it, Firefox won't necessarily become more stable, but it will at least restart when it needs to.