Oh Firefox, Why Can't You Take Out the Garbage?

I am very disappointed in Firefox's memory management.  It's not that Firefox is big and bloated; it actually loads new windows and tabs much fast than Internet Explorer.  However it does a terrible job at garbage collection, which is computer speak for freeing up the memory used by a program or window after it closes.*

I'm a pretty big power user of web browsers and general PC productivity tools (that's right, I did just web browsers a productivity tool).  At any given moment I might have a dozen or more browser tabs open at once, for web applications and sites for communicating, researching, cross-referencing, making plans, etc. 

At the moment my two Firefox windows have 19 tabs:

Therefore I'm not entirely surprised that Firefox is taking up almost half a GB:

or that my general memory utilization looks like this:

I also realize that I'm using a bunch of web applications like Gmail, Google Calendar, Google Reader, Scribd, and of course Blogger, which are probably big memory hogs as compared to static pages.  So, you would think that once I start closing all of these tabs that memory usage would drop dramatically, right?

Wrong.  I've closed everything but Blogger, but my memory usage is still through the roof:

=

Seriously?  I close 18 of 19 tabs and Firefox is still taking up more than 300MB?  I love you with your stability and plug-ins and all, but why can't you be neater like your little brother Chrome?  I'm far enough removed from my programming days that I won't go so far as to say that you have to treat every web page as a separate application, but surely you can do better than this.

* As opposed to waste removal, which is something else entirely.
** I realize that this is not a very precise definition of garbage collection, but it is good enough for present purposes.