Was the file saved from dropbox, then the saved file opened in the browser for viewing? (in firefox: Alt -> File -> Open File)
File hosting sites (drop box, google drive, etc) don't want to be used as web servers, so they do not "serve" html files as "web pages" for browsers to view. This can result in a range of behaviour depending on browser and OS. From the user seeing the html and java script source (which when compressed without white space tends to look like gibberish) to a blank page.