Are there any extensions for Firefox that can automatically hibernate or suspend browser tabs (when they haven't been used for a while?) I saw an extension like this a couple of years ago, but I can't remember its name now.
For Firefox >= 59, there is Auto Tab Discard.
Mozilla is already working on a fork of BarTab to make this a built-in feature. Bugzilla@Mozilla – Bug 675539 Automatically unload (stall/hibernate) longly unused tabs to free RAM
Suspend background tabs: https://github.com/KamiIntelligence/Suspend-background-tabs
For a summary, you can check this reddit thread: https://www.reddit.com/r/firefox/comments/36qc63/which_tab_suspender/
It appears that there is an extension called Tab Mix Plus that does this (by default). It automatically unloads tabs that haven't been used for a while (to free memory), and then allows them to be restored by clicking on them.