As ^ said, FireFox and IE alone aren't multithreaded, so if you have 20 different IE or FF windows open, then it won't be utilizing more then one core. You can check this by going into the task manager, if it stays under 50% with a dual-core, then it's just using one of the cores.
Now if you had FF and IE open, as well as other browsers and other apps, then that would be taking advantage of having a multi-core processor.