high load during Firefox downloads

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http://www.linuxquestions.org – During downloads, the top command shows the Firefox process at 100% CPU. Yesterday I tried to download an .iso image. After a few hours the Firefox window would not refresh nor would it respond to input. I tried the wget command. It used negligible CPU time and completed in 28 minutes. This problem is easy to reproduce because it happens every time I download a file in Firefox. It also happens when I use a fresh profile to run Firefox without any extensions or plugins. Any idea what to try next? This is Firefox 3.6.12 on Fedora 14. (HowTos)