I use a launcher called SlickRun to quickly pull up commonly used applications and websites while I'm working. I recently installed the FoxyProxy plug-in which has since plagued me with strange errors about "DDE" whenever I sometimes open a web address:

The MagicWord eng is broken.
DDE Error.
Would you like to edit the MagicWord?

This only happens if Firefox is not already open. It was more of nuisance than a major issue so I've been ignoring it for a couple of weeks. Today, with the help of a blog post, I was able to solve my issue. Using regedit, I changed the "(Default)" value of key HKEY_CLASSES_ROOT\FirefoxURL\shell\open\ddeexec from "%1",,0,0,,,, to an empty string.

I still wanted to know the root cause, but after more searching I couldn't find much. There are bugs filed with Mozilla and the general idea I get from the comments is that it has to do with Windows' mapping of an HTTP, HTTPS, or FTP requests to a system default application. Some applications require a DDE request and the fix I've listed above just turns it off.