It is worst of all. Just starting it, without opening any pages, already takes 600 Mb. It constantly accesses HDD and stops the world of garbage collection or page unloading or something. How can something be that bad?
Watching a Youtube shouldn't consume more than a few megabytes of RAM at any time, because decoder buffers are of fixed size, yet Firefox manages to leak gigabytes of memory during video playback. That is the pinnacle of broken design.