Hi John, that should never happen unless the web server -- being unaware of the proper Content-Type for a .msi file -- told Firefox it was a "text/plain" or "text/html" file. Unfortunately, it is difficult to think of a way to work around that kind of configuration issue because Firefox doesn't base its download handling decisions on file extensions so much as Content-Type headers.
I created an add-on to help with similar issues, but it isn't pre-configured for MSI because it wasn't something anyone mentioned as a problem. It is possible to add new Content-Type overrides; I have an explanation page in my page here: https://www.jeffersonscher.com/ext/content-type-fixer.html (you'll see it when you get to the "Roll Your Own" section).
