I have no idea - mine uses 184MB. It's been much slimmer than Chrome for me for about a year. Even when I use lots of extensions, it's up to 340MB which is less than Chrome's 400MB.
Even on startup it isn't that better compared to chrome.
Chrome 11 extensions 1 tab: ~300MB
FF 2 extensions 1 tab: ~200MB
It's not FF 29, it's 28 - but I doubt the memory management improved much in 29.
But to be fair, it's a lot better now than it used to be.
But FF is definitely uses more memory than chrome and with few other resource hungry applications running it becomes practically unusable on my low end laptop.