I don't know what kind of PC and/or Firefox you have. I just test it by opening 20 more tabs on different heavy image sites (amazon, 9gag, reddit, twitter, instagram), with some having infinite scrolling and scrolled quite long. The memory jumped from 3.7 GB to 4.7 GB then closed them all. Task manager showed me the memory dropped down back to the original 3.7 GB.
I thought it's obvious is Windows, since I dropped Task Manager as a name there. Or to be absolutely purist, my main OS is Windows under VMWare's hypervisor ESXi, but I have all of them (MacOS, Linux, Android, BSD) as guests on different HDD's/partitions.
I can record a video if you don't believe it.