The article view contents of a QuickBooks IIF file describes how to view an IIF file in your browser. Some browsers will try to use its associated program, QuickBooks, to view the file instead of simply displaying it. Here are some alternatives to view the file.
Use a plain text editor. On a Mac you can use TextEdit. In Windows, Notepad does not display the file properly so you will need to use an alternative. TheGun is a free, small download and does not need to be installed. http://www.movsd.com/thegun.htm (use at your own risk)
View as Text is an add on for Firefox that adds an option to view any downloadable file as text. Just right click the IIF file and select the option to view as text. https://addons.mozilla.org/en-US/firefox/addon/view-as-text/ (use at your own risk)