Firefox has always been known to take more and more RAM. Nonetheless, I find that I've never had a problem with that ever since I've had 4GB of RAM. If you're running a system that uses most of its RAM just with Windows and a few programs running, then Firefox's memory usage might be an issue for you.
Otherwise, it's probably more about the content the open tabs are viewing, the add-ons being used, etc.
A fresh uninstall/reinstall of latest build and check the usage as you add each add-on.... probably the easiest way to pinpoint the issue.