https://addons.mozilla.org/en-US/firefox/addon/bartab-plus/ does the same thing (actually a little better in the implementation) on Firefox. Once you install, go into config for it and set how long a tab may be idle before it auto-unloads. Unloaded tabs are shown slightly dimmed in the tab bar. You can also right click tabs (such as gmail / youtube / etc) and force them to always stay loaded, so your email / music streaming / etc aren't impacted.
Be careful with that because every tab has a constant memory consumption even when not loaded, so it will actually have a noticeable memory consumption when you have several hundreds of tabs. I learned it the hard way.
Combo it with https://addons.mozilla.org/en-US/firefox/addon/tab-groups-pa... that lets you organize your tabs into groups and you can have hundreds of well organized tabs without using much memory at all.