That happens to me and there's nothing I can do about it. And some websites only work in Firefox.
Firefox is bad because it is based on Netscape making it's core bad.
Basically on Microsoft Windows, when you close a program expecting your pc to speed up and get more ram, sometimes you will find that this does not happen. And you have to wait a couple of minutes for the cpu and memory to go down. This is called a memory leak due to bad memory management and can be fixed with MemOptimizer. Anyway, this same thing is happening to Firefox. I use Opera.