It is possible that only the file extension is wrong, so you can try to rename the files and give them the right file extension based on what you would expect to see if that helps. You can also inspect <b>handlers.json</b> in the Firefox profile folder and check if there is an entry for ".xls". You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the <b>about:profiles</b> page (Root directory). *Help -> More Troubleshooting Information -> Profile Folder/Directory:<br>Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder *https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data