Dormancy 'retires' tabs that have gone unused for a while, freeing up that memory. It then restores the tabs to life when accessed.

Tabs are considered inactive when they haven't been selected in longer than 10 minutes.

Tabs are checked for inactivity every 10 minutes.

Install: https://addons.mozilla.org/en-US/firefox/addon/dormancy/

Note:

• Your form data should not be lost. This extension uses underlying Firefox APIs to save/restore the tabs, and those APIs handle form data. If you're seeing form data lost, please file a bug for Firefox: https://bugzilla.mozilla.org/enter_bug.cgi?product=Firefox