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.

For users

• 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.

For webmasters

Read Properly Configuring Server MIME Types.

See also

Download shows as gibberish