Tabinator by ScienceDiscoverer

This is more lightweight and to-the-point solution to saving system memory.
This add-on will be extremely useful for people like me, who often tend to have 300 or more tabs opened and keep adding more. Firefox can take up to 10 GB or more RAM in this cases. If you have 32 GB+ of memory, automatic tabs unloading won't trigger, so the only solution to this problem is to use the plug-in. It actually uses exactly the same API that built-in tabs unloading uses - tabs.discard().

Compared to other alternatives, the benefit of this one is that it only does what it need to do - unloads (discards) tabs that was not activated in a set amount of time. And nothing more. It runs only with a set interval, so it won't waste a lot of CPU time.

Internally it also keeps track of the number of opened tabs, without polling it every time (simple counters). It will stop if amount of tabs is less than specified in options page.

Pinned tabs are always ignored, so if you have some important tabs you don't want to be unloaded ever, pin them.