I also found out something interesting while using tabs. If I open one IE brower it uses approx 11.7MB of RAM to open Google.com, while one instance of Firefox takes 17.1 MB. But what if I want to view 2 pages at once (which is almost always the case)?
If I open up each page in a new Firefox window, each new page adds about 1 MB. If I use tabs it takes up about 150k for each new opened Google.com page. But if I used IE, each page takes 11.7 MB!!! I tested both Apps (Firefox tabbed and windowed).