Firefox is known to be an insane memory hog-with only 5 tabs open it can take up over 141,000 K of memory. At least that's what my computer says in Task Manager.
But even then... you have 4 GB of memory, which shouldn't be a problem. Have you done any virus scans recently? Are there any background processes which you have forgotten to close? Does the CPU usage rise to 100% only when you are in Firefox?
I noticed that you have a YouTube tab open. Flash is CPU intensive. Is your CPU usage still at 100% even if you are not on YouTube?
Try a different browser, such as Google Chrome or Opera, and see what happens...