I had been seeing increasing incidences of this. It was getting very annoying. FWIW, I'm using FF 6.0 and 10.6.8. This wasn't where FF was just taking time downloading. I'd get the spinning beach ball for 15-20 seconds BEFORE it started actually downloading, and frequently when I was already on a web page, and just scrolling down. I tried disabling all plug-ins and extensions. No help. Here's how I fixed it. (1) Fix permissions and (2) Completely remove and restore Firefox. Remove all FF preferences (trash user/Library/Firefox), the application, and re install the application. Yes, be sure you preserve your bookmarks, ideally through Xmarks, and you'll have to redo all your passwords. Small price to pay. After I did this, I had NO problems. Spinning beach balls are gone. But I'd sure like to know why ...