What site is this?

This could occur if the website mistakenly identifies the file as a PDF using Content-type: application/pdf However, that would be pretty strange. It's more common for websites not to have any specific Content-Type (also known as MIME Type) for CSV files, and to identify them either as plain text or as generic binary content. Occasionally sites will improvise a Content-Type that Firefox has seen before and associated with a different file extension, and this *could* cause Firefox to wrongly assume that the download's file extension needs to be corrected. You can turn off the file extension fix-up feature for non-media files and see whether that makes any difference. Here's how: (1) In a new tab, type or paste '''about:config''' in the address bar and press Enter/Return. Click the button accepting the risk. ''More info on about:config:'' [[Configuration Editor for Firefox]]. ''The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.'' (2) In the search box in the page, type or paste '''browser.download.sanitize_non_media_extensions''' and pause while the list is filtered (3) Double-click the preference to switch the value from true to false Then test the download again and see whether Firefox preserves the exact file name assigned by the site. Did that make any difference?