This cloud-based add-on that turns your browser into a universal file viewer. Open 500+ file types with Firefox. Anywhere. Anytime.
Here is more information from the developer:
View online virtually any file that you might find while surfing the Internet without downloading a file to your computer or installing any applications. Just click the file and it will open instantly in a fully featured application running in your browser. If you have Dropbox or Google Drive you can save changes to the file right to a cloud-storage of your choice (Box and 4shared are also supported).
This is a really revolutionary add-on for Firefox. You can pick up rollApp File Opener from the Firefox Add-ons website.