Giving that 500Mb is not much for any remotely modern PC, and the fact that FF consumes massively less memory than Chrome, I just don't see a problem.
While not everyone is able to use or own a computer with 8GB RAM or more - which is about what's needed for some serious multi-tasking - I can absolutely agree with your point.
The maximum memory efficiency after a number of tabs were closed, doesn't play as large a role in the browser or system performance as people would expect. At least that's the way I see it. It's perfectly all right to slam a browser for memory leaks, but Firefox got rid of most of them in the post 3.5 release stream. I haven't experienced a memory leak in months now and even that was caused by an extension and not the browser itself.
If Firefox' only criticism should be, that the browser doesn't recapture the minimal use of memory it had after a fresh start with only 1 tab, then I don't care! To me it's much more critical, that Chrome takes double the memory for the same amount of tabs while they're being used
. Here's an example statistic from me with both figures taken from Chrome's accurate about:memory page:
Firefox 22: 5 tabs active, 87 unloaded = 357MB RAM
Chrome 28: 4 tabs active = 774MB RAM
The browsers are directly comparable in their use of extensions, as Adblock Plus, Ghostery, Stylish and Grease/Tampermonkey are loaded in both browsers, Firefox even runs half a dozen more.
I'd much rather restart Firefox manually every now and then - which takes me less than 15 seconds, as tabs remain unloaded when the session is restarted - because Chrome's only salvation on a lower class computer would be to add more RAM to the computer.