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.
It's not just a programming environment, either; a web browser is an interpreter for some of the most poorly-written, highest-level code in existence. Not very conducive to good memory usage even theoretically.
But then the Ctrl+Shift+T feature to reopen closed tabs wouldn't work so well. Try it, you can reopen closed tabs as far back as you like; it's pretty awesome and has saved me more than once.
It doesn't work in Private Mode (for good reason); I wonder if browsing in private mode consumes less memory?
I like that feature and use it. But it can be separated from the memory caching. Back button speed is nice too, but I could do without it.
There are a few options here ... keep the last one in memory, the last few in the disk cache, etc. Right now it appears to keep everything you've ever visited in RAM, which is clearly wrong.
I believe that there are also some information-theoretical limits on the efficiency of garbage collection and memory allocation. Don't quote me on that, though.
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.