That may be true with a small session, or a minimal number of open tabs. Yet, with a large session|many tabs, FF becomes unresponsive regularly (CPU Spikes) and there's almost nothing you can do to make it release RAM, except for closing the browser. FF's CPU usage also spikes on launch 30-50% on a quad core with a large session, even when only a single tab of a given window is loaded on launch. The CPU usage also spikes whenever you manage tabs (move|close).
Compared to almost any other Browser FF lags badly in terms of resource management, including IE11, and Blink-based browsers (Chrome, Opera, etc). It also doesn't seem to matter what branch of FF you use, they all are horrible at resource management (FF Nightly 32bit or 64, WaterFox/64bit, FF Dev 32bit (previously Aurora).
And yet there are lots of Firefox users, like me, who never experience the issues you describe. Firefox is my main browser. I use it heavily all day, every day, and haven't had memory or CPU problems in years. I suspect that a lot of the reported Firefox resource issues have more to do with the combination of extensions/plugins that people have installed than Firefox itself.