I certainly hope that memory is a function of how much content is loaded because one would expect a modern desktop application to keep all its non-persistent working data in memory, and not try to outsmart the OS when it comes to memory management. After all, RAM is cheap, your OS is perfectly capable of managing virtual memory, and addressable memory isn't a problem on 64 bit systems.
The irony is that one of the reasons why modern mobile OS often feel faster than desktop applications on much faster hardware is because their APIs force programmers to give up this kind of hubris. So basically, Mozilla is building FirefoxOS, which will enforce the reasonable behavior they aren't capable of themselves when in the role of an application developer.
I think RAM usage should be a function of work load. However, here I sit with 12 tabs open and Firefox using almost 1GB of RAM. I don't know when this became okay, but I could see Firefox using 4GB of RAM, easy. At this rate, that would be due to the ungodly workload of 48 tabs.
The irony is that one of the reasons why modern mobile OS often feel faster than desktop applications on much faster hardware is because their APIs force programmers to give up this kind of hubris. So basically, Mozilla is building FirefoxOS, which will enforce the reasonable behavior they aren't capable of themselves when in the role of an application developer.