I've easily gotten it up to 4GB. Five windows, several tens of tabs per window. Webapps like GMail (2 instances of it), GCal (also 2), Facebook, Tweetdeck, Slack, etc. open all the time eat a ton of RAM, and just various complex websites/apps sitting open over time will often leak a bit.

I just restarted FF an hour or so ago, and haven't even had it reload most of the tabs that used to be open (FF restored them, they're just not loaded yet as I haven't tabbed to them), and I'm already at 1.5GB resident with probably 15 active/loaded tabs.