Firefox had been giving me the spinning beach ball of doom for weeks, every time I downloaded a file. Whether a 5k GIF or 50MB FLAC, the browser hung for 30-40 seconds before finally popping the Downloads window open.
You’d think it would take a pretty serious computing challenge to redline two 2GHz G5 CPUs, but no — just try downloading some album art from Google Images. Apparently there is some kind of CPU-cooking fractal math in the progress-bar code in the Downloads window. Or maybe Firefox is calling into the SETI@home project on the sly, scanning a few parsecs of some remote slice of the universe for intelligible radio signals before ultimately beginning the admittedly pedestrian task of saving a few hundred bytes of image data to my hard drive.
Thanks go to Karl Pietri for suggesting I click the “clean up” button in the Downloads window. Erasing the past year’s worth of download history cut the download startup time from 30+ seconds to a reasonable .05 seconds.
More info, and a screenshot, can be found here: Firefox Download Clean Up