"Was there a particular upgrade that addressed process management and/or memory leaks? I also wonder if it's an OS thing (I'm on OS X)."

I'm on Windows 7, and in my experience the memory issues in Firefox haven't been completely resolved yet (though the Mozilla team are actively working on it). However, as a workaround I have found the Memory Fox add-on useful: https://addons.mozilla.org/en-US/firefox/addon/memory-fox/