[arch-general] Something wrong with firefox/thunderbird driving X cpu usage -> 100%

Guys, I have watched this in top for the past week or so. Thunderbird and Firefox are causing X cpu usage to shoot upt to between 80-100% on simple tasks like scrolling a message list in tbird or simply opening css menus in firefox. All of this used to be instantaneous and never cause the cpu to bat an eye. But now, it is very pronounced and brings the desktop to a crawl. This box is not a screamer, but plenty fast, P4 2800/4G/Nvidia 8600GT. How do I determine what is causing this? X? ff/tb? something else? Any ideas appreciated. Thanks. -- David C. Rankin, J.D.,P.E.