I restored from an image file I made before I clicked the ed2k link and found the problem was a windows update that was installed.
It's one (or both) of the follwing:
Update for Windows XP (KB898461)
This update installs a permanent copy of Package Installer for Windows to enable software updates to have a significantly smaller download size. The Package Installer facilitates the install of software updates for Microsoft Windows operating systems and other Microsoft products. After you install this update, you may have to restart your system.
Update for Windows XP (KB910437)
Install this update to prevent or resolve an issue in which Windows Update and Automatic Updates can no longer download updates after an Access Violation error occurs when using the Automatic Updates
service. After you install this item, you may have to restart your computer.
It may be another one of the updates (there were 16) since uninstalling these 2 affects some of the others. I'm surprised no one else has this problem. My solution is to leave the updates on my system and just use Free Download Manager or IE to download exe files. Maybe Mozilla will find and fix this issue in a future release. The .part file extension is actually a Firefox incomplete download file extension.
>> Stay informed about: can't download EXE files with firefox