Crazy. My instance has been running for days, with dozens of tabs open at once and I am currently at 89Mb with 4 tabs open at the moment i.e. it scales back the memory like it should. I've found that memory usage has improved significantly in recent builds.

If you're using more than a GB under normal conditions there's either something wrong with one of your extensions or your profile is messed up. Try starting a new profile with no extensions and see if the problem remains.