The key word here is "workload". A web browser is a programming environment. It would be silly to say "no single native program should use 500MB of RAM", and it's equally silly to say that no website (and by extension, the browser) should use that much.
That's not to say Firefox's memory usage is perfect, far from it. That's why we're making improvements like this one. See https://wiki.mozilla.org/Performance/MemShrink for more.