Does the file work if you rename the file and add a .pdf file extension?

If that works then you will have to make sure that you give the file the correct file extension when you save the file when you get the open with download dialog. The website may not be using the correct way to specify the file name when you retrieve (save) the file.

Since the file was already saved, you can rename it (right-click > Rename). By default, Windows makes it a little difficult to change file extensions but if you set Windows to show ALL extensions, it will be easier. See: http://windows.microsoft.com/en-us/windows/show-hide-file-name-extensions.

Going forward, if the server isn't send a PDF file name that Firefox understands (.do is the common page ending for the Lotus Domino server), you could set Firefox to ask you where to save downloads so you can fix the .pdf extension before saving. That setting is on the Options page. This article has more information: Startup, home page, tabs, and download settings.

Does that help?

No, cannot rename. Can only rename the file and not the extension. And i already have Windows set to show ALL extensions. When I went to where Firefox asks to save downloads, it still will not allow me to change the extension to .pdf at all. I have Windows 10, is that the problem?

Windows 10 still should have the ability to change any part of the file name, including the extension.

The change to Firefox -- showing the Save As dialog for downloads -- should let you rename the file before saving for future downloads, but would not be retroactive to past downloads.