They're both ridiculous. I know browsers are really feature-intensive nowadays, but really, 500mb for a few tabs?
I think the story is that Firefox uses less, but because Chrome has an individual process for each tab they don't all die at once, and Chrome has a nicer about:memory page.
I've tried using lighter browsers like Midori but can't get away from having 10+ extensions for various things.
Don't get me wrong, I use Firefox and I love it, it's my main browser. It was just a simple joke with no malicious intents.
That said and done, I've had firefox run for several days on my computer and reach up to 3-4GB of memory used (I have 8GB). At that point it becomes very clunky and requires a restart.
I have 32GB ram so I can cope, but calling out Firefox as the worst offender in class is a wee bit rich.