Excel application doesn't exist under Option tab

http://dbissues.blogspot.com/2016/06/excel-application-doesnt-exist-under.html

Try renaming your localstore.rdf mimetypes.rdf file in your profile folder. Note that this will reset your application preferences.

Go to your troubleshooting page in Firefox: 

1. Enter about:support in the address bar, or go to Help (or > ) > Troubleshooting Information
2. Click the "Show Folder" button
3. See Profiles - Where Firefox stores your bookmarks, passwords and other user data for more information

This should open your profile folder in Windows Explorer. Close Firefox and then rename the localstore.rdf mimetypes.rdf file to something else, such as localstore.old mimetypes.old . Start Firefox again and it will create a new localstore.rdf - try opening an Excel file in Firefox now. If it works properly, you can remove the localstore.old mimetypes.old or whatever you renamed the file as.