Why does firefox make my computer slow?

So I recently downloaded firefox cause i heard alot of great things about it and wanted to try it. But since i downloaded it my computer began to get slower and slower and more laggy. IT would take 2 minutes open a large file. So i got fed up trying to find the problem and i installed firefox and switched to IE. Instantly my computer was sped up and much faster and back to normal. WTF?