Why won't this Word Document download/open in Firefox?

Question?

I'm trying to open MS Word documents that my students uploaded to fulfill an assignment, but I can't get the files to open or download. I am using Firefox. Do you know what's happening?

Answer!

Sometimes Firefox will be configured to try to open a file when you download it, but won't know which application to use to open a file. Sometimes it even tries to open the file in Firefox itself—even if it isn't a file type that it can handle. You can fix its behavior by setting it up to use the proper application to open a given file type.

This Firefox help article describes how to do so.