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).
I really doubt the shrinking user-base has much if anything to do with Australis either, it's pretty easy to add back the Status Bar, Addon Bar, or any number of bars that you want.
Although personally, I wonder why the hell they (Mozilla) don't distribute some of their Devs or money ($120+ Million for FF Development in 2012) to some of the MAJOR Addon authors and help them get on board with Electrolysis (e10s). I imagine FF's user-base will shrink even further before (or if) Mozilla ever sees the light.