Firefox usually ignores the file extension and bases decisions about what to do with a file on a content type header (also known as MIME type) sent by the web server. This isn't always sensible information, unfortunately. You can check your default setting here: orange Firefox button ''or'' Tools menu > Options > Applications In the search box type '''csv''' and pause to see whether two Excel-related listings come up. Mine are set to "Always ask" but yours may vary or be missing. If they are completely missing, you might want to reset your download actions as described in this article: http://kb.mozillazine.org/File_types_and_download_actions#Resetting_download_actions If Firefox is displaying the CSV files from some sites as though they are ordinary web pages, you may need to use right-click > Save Link As to save the file locally before launching it in Excel.