This is normal behaviour for Firefox. It's just it's way of letting you know it's doing something. As far as I know you can neither remove or turn off this action. I have always found Firefox to be slow. If you want to use Firefox I'm afraid you'll just have to go with the flow.
There are a few small tweaks to speed up Firefox although I can't remember them off hand. I tend to install and forget. If I can think of them I'll post them.