From MozillaZine Knowledge Base
If you are trying to view a file from a site and instead it prompts for a download location, it's most likely the result of a misconfigured server.
- Contact the webmaster of the site and tell them about the issue. This is very important so that not only you can view the file properly, but others can too.
- If you must view the page, you can download the file, rename it to the proper extension, then open it locally.
Read Properly Configuring Server MIME Types.
Download shows as gibberish