Opening too many Firefox tabs and keep them visible all the time can lead to a ridiculously huge memory consumption, which is why you may need an extension that hands you the ability to unload tabs either manually or automatically. And this one, Auto Unload Tab, is probably the extension that you’ve been probably looking for.
By default the extension unload tabs that have been inactive for 5 minutes automatically. Additionally, any URL that you open in background tab is kept unloaded until you select it.
It also limits the max number of loaded tabs up to 3 tabs. In other words, when you open more than 3 tabs, one of the existing tabs will be unloaded with the exception of the ones that you choose to keep loaded.
If you like to do things on your own and thus disable the auto unload, or if you simply need to unload a certain tab immediately, you can do so by middle-clicking the tab or by right-clicking the tab and select “Unload Tab” from the menu.
All of these options are customizable. Just go to the extension’s Options panel to make changes according to what you need.
And once you open its Options panel, you’ll see that it has more under its sleeves.
For example, you can choose to auto close tabs that have been inactive for a certain customizable length of period, unload pinned tabs, prevent unread tabs from being automatically unloaded, prevent tabs from being auto reloaded when you load them, and so on.
Auto Unload Tab 0.6.7 is a restartless Firefox add-on. You can get it from here.