Auto Tab Reaper от Chris Trotman

This extension automatically discards tabs using the built in Firefox discard functionality.
It will by default skip all pinned tabs and all audible tabs, however you can change this in the addon preferences.
You can also change from the default 30 minute delay to 1, 5 and 10 minute delays before suspending tabs.

Source code is available at https://github.com/solarnz/auto-tab-reaper