The behaviour has been built in and accessible since Firefox 4 (initially through about:config, later through the Preferences), but has only been enabled by def

The behaviour has been built in and accessible since Firefox 4 (initially through about:config, later through the Preferences), but has only been enabled by default later on. I've personally been using it since it appeared, and it's great if you literally have hundreds of tabs open, or use tabs as temporary bookmarks.