The message it gives is that the DOM Inspector is not installed…except that it was in my case.  So that was pretty perplexing.  However, Firebug itself points you to an FAQ which in turn points you to a forum entry.  The grammar isn’t great, but the basic steps are:

1. Find your Mozilla directory within your application data directory and make a backup of it.
2. Go to the Control Panel and remove Firefox from your programs list.
3. Go to the directory where Firefox used to be fully installed and remove it (the uninstall doesn’t do it for you).
4. Download and install Firefox, but in a different directory than the original installation.
5. Restore your application data if it has been removed.

Some of these steps make no sense, but I followed them blindly and it worked.  I’m pretty sure that the process can be streamlined, but since I’m up and running I’ll leave that to someone else.  Since this was hard to find and harder to understand once found, I thought I would write it up for anyone who might need to do the same thing.