Idle Tab Manager от John

Highly configurable extension for automatically unloading / discarding tabs, to increase your battery life and reduce CPU / memory load.

By default, the 3 most recently viewed tabs will not be discarded, and tabs that have not been viewed for more than 20 minutes will be discarded. The only exception enabled by default is for tabs playing audio.

Configurable options:
* Set the check interval (minutes), tab idle timeout (minutes), and the number of recent tabs to keep alive.
* Add regex patterns for tab titles and / or URLs to exclude from auto-discarding.
* Toggles for whether audible / pinned tabs should be excluded from auto-discarding.

Supports Firefox, Zen Browser, and probably other forks as well.